Assays and methods of treatment relating to vitamin d insufficiency

ABSTRACT

Described herein are assays directed to determining the level of bioavailable or free vitamin D in a blood sample in a subject. The values determined for bioavailable or free vitamin D indicate whether the subject suffers from insufficient levels of vitamin D. Also described herein are methods of treatment for vitamin D insufficiency.

CLAIM OF PRIORITY

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/819,235, filed on May 3, 2013, and U.S. patentapplication Ser. No. 13/918,563, filed on Jun. 14, 2013. The entirecontents of the foregoing are hereby incorporated by reference.

GOVERNMENT SUPPORT

This invention was made with federal government support under GrantNumber M01-(RR-01066)-Harvard Clinical and Translational Science Centerawarded by the National Center for Research Resources and Grant Nos. K231K23DK081677 and R01 DK 094486 awarded by the National Institutes ofHealth. The U.S. Government has certain rights in the invention.

FIELD OF THE INVENTION

The invention is directed to assays and methods of treatment relating tovitamin D insufficiency.

BACKGROUND OF THE INVENTION

Vitamin D insufficiency (variously defined as <20 to 30 ng/mL of totalserum 25(OH)D as currently measured) is highly prevalent, even inotherwise healthy individuals. Reported in >1 billion people worldwide,it is now recognized as one of the most common subclinical medicalconditions in the world. Beyond rickets, a manifestation of severevitamin D insufficiency recognized since the 17th century,³⁵ vitamin Ddeficiency (commonly defined as <10-25 ng/mL of total serum25[OH]D)^(21,34,36) has since been associated with an increased risk ofosteoporosis, cancer, infectious disease, CVD disease, allergy, asthma,multiple sclerosis, muscle weakness, rheumatoid arthritis, and diabetes.Low vitamin D can arise from insufficient intake from nutritionalsources, insufficient synthesis (via UV-B radiation of the skin),adiposity, age, physical activity, or other disease-related factors suchas diabetes, bariatric surgery, fat malabsorption syndromes, and kidneydisease.^(14,37,38) The use of sunscreen with sun protection factor(SPF)≧30 reduces the ability of the skin to produce vitamin D by 99%,thus contributing to the pandemic.

One recent study found that vitamin D insufficiency was present in 72%of community-dwelling men older than 65 years of age, and in up to 86%of those men who were obese, lived at higher latitudes, or infrequentlyparticipated in outdoor activities.¹⁷ Although vitamin D deficiency isless common, it is estimated to affect 26%-54% of community-dwellingolder adults and 57% of hospitalized patients.^(17,36,40) A recognizedproblem in older adults, people of all ages who live in diversegeographic locations are also susceptible, including sunny climatedwellers.⁴¹ A study of younger adults with limited exposure to theoutdoors in a northeastern urban setting reported that 32% of studentsand doctors aged 18-29 years were vitamin D deficient at the end of thewinter.⁴² In diseases including diabetes, rheumatoid arthritis, renaldisease, as well as in individuals who are obese, hospitalized,pregnant, newborn, highly deficient levels of this hormone arecommon.^(40,43-45) Current recommendations for vitamin D supplementationare largely inadequate.^(17,46,47) According to Holick,⁴⁶ 25(OH)D is themost-ordered hormone assay in the US, used as the basis for treatmentrecommendations. However, assay results as well as cutoff levels of25(OH)D to define the extent of vitamin D insufficiency are subject toconsiderable variation. Given the prevalence and breadth of illnessespotentially associated with low vitamin D, gaining a betterunderstanding of vitamin D status to guide management of vitamin Dinsufficiency is a public health priority. The Institute of Medicine(IOM) has recognized that that assay variation and lack of consensusregarding cutoffs defining insufficiency/deficiency have causedconfusion about the appropriate dietary intake of vitamin D.³³ The IOMhas also cautioned against excessive intake due to the risks of kidneyand tissue damage and have urged more targeted research in this area.Importantly, the method used to determine vitamin D status should beclinically relevant and applicable across diverse populations.

SUMMARY OF INVENTION

Described herein are methods, assays, methods of treatment, and systemsrelated to determining the level of free and/or bioavailable vitamin Din a blood sample obtained from a subject.

In one aspect, the invention relates to an assay comprising a) analyzinga blood sample obtained from a subject to determine a level of VDBP(vitamin D binding protein) polypeptide, albumin polypeptide and totalvitamin D; wherein a level of bioavailable vitamin D is:

=(K _(alb)*[Alb]+1)*[Free Vitamin D]

and wherein a level of free vitamin D is:

={−{K _(DBP)·[Total DBP]−K_(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1}+√{(K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1)²+4·(K _(DBP) ·K _(alb)·[Alb]+K _(DBP))·([Total VitaminD])}}÷(2·{K _(DBP) ·K _(alb)·[Alb]+K _(DBP)}).

In some embodiments, a level of bioavailable vitamin D lower than athreshold level, e.g., the 25th percentile value or 25% of the meanvalue, of bioavailable vitamin D in a population of healthy subjects canindicate that the subject has a vitamin D insufficiency. In someembodiments, the vitamin D can be selected from the group consisting of:

25-hydroxyvitamin D and 1,25-dihydroxyvitamin D.

In another aspect, the invention features methods for determining aVitamin D Index for a subject. The methods include determining total25(OH)D concentration in the subject; determining a VDBP concentrationin the subject; and calculating (e.g., using a processor as known in theart and described herein) a ratio of total 25(OH)D concentrationsdivided by VDBP concentrations, thereby determining the Vitamin D Indexfor the subject.

In some embodiments, % bioavailable radioligand (defined by the ratio ofadsorbed radioligand divided by total radioligand) is converted intocalculated % bioavailable 25(OH)D, e.g., using a calibrator standardcurves (e.g., as in FIG. 12). The concentration of bioavailable 25(OH)D(in ng/ml) in a patient sample is then obtained by multiplying theirmeasured total 25(OH)D concentrations by the calculated % bioavailable25(OH)D.

In some embodiments, the methods include determining a VDBP genotype inthe subject; and comparing the Vitamin D Index to a reference Index forthe subject's genotype; wherein the presence of a Vitamin D Index belowthe reference Index indicates that the subject has a vitamin Dinsufficiency.

In some embodiments, determining the level of VDBP polypeptide oralbumin polypeptide can comprise the use of a method selected from thegroup consisting of: enzyme linked immunosorbent assay; chemiluminescentimmunosorbent assay; electrochemiluminescent immunosorbent assay;fluorescent immunosorbent assay; dye linked immunosorbent assay;immunoturbidimetric assay; immunonephelometric assay; dye-basedphotometric assay; western blot; immunoprecipitation; radioimmunologicalassay (RIA); radioimmunometric assay; immunofluorescence assay and massspectroscopy.

In some embodiments, determining the level of total vitamin D cancomprise the use of a method selected from the group consisting of:radioimmunoassay; liquid chromatography tandem mass spectroscopy; enzymelinked immunosorbent assay; chemiluminescent immunosorbent assay;electrochemiluminescent immunosorbent assay; fluorescent immunosorbentassay; and high-pressure liquid chromatography.

In some embodiments, an insufficiency of vitamin D can indicate anincreased risk of a condition selected from the group consisting of:decreased bone density; decreased bone mineral density; bone fractures;bone resorption; rickets; osteitis fibrosa cystica; fibrogenesisimperfect ossium; osteosclerosis; osteoporosis; osteomalacia; elevatedparathyroid hormone levels; parathyroid gland hyperplasia; secondaryhyperparathyroidism; hypocalcemia; infection; cancer; psoriasis;cardiovascular disease; renal osteodystrophy; renal disease; end-stagerenal disease; chronic kidney disease; chronic kidney disease-associatedmineral and bone disorder; extraskeletal calcification; obesity;allergy, asthama; multiple sclerosis; muscle weakness; rheumatoidarthritis and diabetes.

In some embodiments, the invention can further comprise the step ofadministering a vitamin D insufficiency treatment to a subject who isdetermined to have a vitamin D insufficiency. In some embodiments, thetreatment can comprise administering a compound selected from the groupconsisting of: calcitriol; dihydrotachysterol; doxercalciferol;paricalcitol; cholecalciferol and ergocalciferol.

In another aspect, the invention relates to an assay comprising;analyzing a blood sample obtained from a subject to determine a level ofVDBP polypeptide, albumin polypeptide and total vitamin D; wherein alevel of free vitamin D is:

={−{K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1}+√{(K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1)²+4·(K _(DBP) ·K _(alb)·[Alb]+K_(DBP))·([Total VitaminD])}}÷(2·{K _(DBP) ·K _(alb)·[Alb]+K _(DBP)}).

In some embodiments, a level of free vitamin D lower than a thresholdlevel, e.g., the 25^(th) percentile value or 25% of the mean value, offree vitamin D in a population of healthy subjects can indicate that thesubject has a vitamin D insufficiency. In some embodiments, the vitaminD can be selected from the group consisting of: 25-hydroxyvitamin D and125-dihydroxyvitamin D.

In some embodiments, determining the level of VDBP polypeptide oralbumin polypeptide can comprise the use of a method selected from thegroup consisting of: enzyme linked immunosorbent assay; chemiluminescentimmunosorbent assay; electrochemiluminescent immunosorbent assay;fluorescent immunosorbent assay; dye linked immunosorbent assay;immunoturbidimetric assay; immunonephelometric assay; dye-basedphotometric assay; western blot; immunoprecipitation; radioimmunologicalassay (RIA); radioimmunometric assay; immunofluorescence assay and massspectroscopy.

In some embodiments, determining the level of total vitamin D cancomprise the use of a method selected from the group consisting of:radioimmunoassay; liquid chromatography tandem mass spectroscopy; enzymelinked immunosorbent assay; chemiluminescent immunosorbent assay;electrochemiluminescent immunosorbent assay; fluorescent immunosorbentassay; and high-pressure liquid chromatography.

In some embodiments, an insufficiency of vitamin D can indicate anincreased risk of a condition selected from the group consisting of:decreased bone density; decreased bone mineral density; bone fractures;bone resorption; rickets; osteitis fibrosa cystica fibrogenesisimperfect ossium; osteosclerosis; osteoporosis; osteomalacia; elevatedparathyroid hormone levels; parathyroid gland hyperplasia; secondaryhyperparathyroidism; hypocalcemia; infection; cancer; psoriasis;cardiovascular disease; renal osteodystrophy; renal disease; end-stagerenal disease; chronic kidney disease; chronic kidney disease-associatedmineral and bone disorder; extraskeletal calcification; obesity;allergy, asthama; multiple sclerosis; muscle weakness; rheumatoidarthritis and diabetes.

In some embodiments, the invention can further comprise the step ofadministering a vitamin D insufficiency treatment to a subject who isdetermined to have a vitamin D insufficiency. In some embodiments, thetreatment can comprise administering a compound selected from the groupconsisting of: calcitriol; dihydrotachysterol; doxercalciferol;paricalcitol; cholecalciferol and ergocalciferol.

In another aspect, the invention relates to a method for treating avitamin D insufficiency in a subject comprising detecting a level ofVDBP polypeptide, albumin polypeptide and total vitamin D in a bloodsample obtained from a subject; wherein a level of bioavailable vitaminD is:

=(K _(alb)*[Alb]+1)*[Free Vitamin D]

and wherein a level of free vitamin D is:

={−{K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1}+√{(K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1)²+4·(K _(DBP) ·K _(alb)·[Alb]+K _(DBP))·([Total VitaminD])}}÷(2·{K _(DBP) ·K _(alb)·[Alb]+K _(DBP)}).

and administering a treatment for vitamin D insufficiency to the subjectif the level of bioavailable vitamin D is lower than a threshold level,e.g., the 25th percentile value or 25% of the mean value, ofbioavailable vitamin D in a population of healthy subjects.

In some embodiments, the vitamin D can be selected from the groupconsisting of: 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D.

In some embodiments, determining the level of VDBP polypeptide oralbumin polypeptide can comprise the use of a method selected from thegroup consisting of: enzyme linked immunosorbent assay; chemiluminescentimmunosorbent assay; electrochemiluminescent immunosorbent assay;fluorescent immunosorbent assay; dye linked immunosorbent assay;immunoturbidimetric assay; immunonephelometric assay; dye-basedphotometric assay; western blot; immunoprecipitation; radioimmunologicalassay (RIA); radioimmunometric assay; immunofluorescence assay and massspectroscopy.

In some embodiments, determining the level of total vitamin D cancomprise the use of a method selected from the group consisting of:radioimmunoassay; liquid chromatography tandem mass spectroscopy; enzymelinked immunosorbent assay; chemiluminescent immunosorbent assay;electrochemiluminescent immunosorbent assay; fluorescent immunosorbentassay; and high-pressure liquid chromatography.

In some embodiments, an insufficiency of vitamin D can indicate anincreased risk of a condition selected from the group consisting of:decreased bone density; decreased bone mineral density; bone fractures;bone resorption; rickets; osteitis fibrosa cystica; fibrogenesisimperfect ossium; osteosclerosis; osteoporosis; osteomalacia; elevatedparathyroid hormone levels; parathyroid gland hyperplasia; secondaryhyperparathyroidism; hypocalcemia; infection; cancer; psoriasis;cardiovascular disease; renal osteodystrophy; renal disease; end-stagerenal disease; chronic kidney disease; chronic kidney disease-associatedmineral and bone disorder; extraskeletal calcification; obesity;allergy, asthama; multiple sclerosis; muscle weakness; rheumatoidarthritis and diabetes.

In some embodiments, the treatment can comprise administering a compoundselected from the group consisting of: calcitriol; dihydrotachysterol;doxercalciferol; paricalcitol; cholecalciferol and ergocalciferol.

In another aspect, the invention relates to a method for treating avitamin D insufficiency in a subject comprising detecting a level ofVDBP polypeptide, albumin polypeptide and total vitamin D in a bloodsample obtained from a subject; wherein a level of free vitamin D is:

={−{K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1}+√{(K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1)²+4·(K _(DBP) ·K _(alb)·[Alb]+K _(DBP))·([Total VitaminD])}}÷(2·{K _(DBP) ·K _(alb)·[Alb]+K _(DBP)}).

and administering a treatment for vitamin D insufficiency to the subjectif the level of free vitamin D is lower than a threshold level, e.g.,the 25th percentile value or 25% of the mean value, of free vitamin D ina population of healthy subjects.

In some embodiments, the vitamin D can be selected from the groupconsisting of: 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D.

In some embodiments, determining the level of VDBP polypeptide oralbumin polypeptide can comprise the use of a method selected from thegroup consisting of: enzyme linked immunosorbent assay; chemiluminescentimmunosorbent assay; electrochemiluminescent immunosorbent assay;fluorescent immunosorbent assay; dye linked immunosorbent assay;immunoturbidimetric assay; immunonephelometric assay; dye-basedphotometric assay; western blot; immunoprecipitation; radioimmunologicalassay (RIA); radioimmunometric assay; immunofluorescence assay and massspectroscopy.

In some embodiments, determining the level of total vitamin D cancomprise the use of a method selected from the group consisting of:radioimmunoassay; liquid chromatography tandem mass spectroscopy; enzymelinked immunosorbent assay; chemiluminescent immunosorbent assay;electrochemiluminescent immunosorbent assay; fluorescent immunosorbentassay; and high-pressure liquid chromatography.

In some embodiments, an insufficiency of vitamin D can indicate anincreased risk of a condition selected from the group consisting of:decreased bone density; decreased bone mineral density; bone fractures;bone resorption; rickets; osteitis fibrosa cystica; fibrogenesisimperfect ossium; osteosclerosis; osteoporosis; osteomalacia; elevatedparathyroid hormone levels; parathyroid gland hyperplasia; secondaryhyperparathyroidism; hypocalcemia; infection; cancer; psoriasis;cardiovascular disease; renal osteodystrophy; renal disease; end-stagerenal disease; chronic kidney disease; chronic kidney disease-associatedmineral and bone disorder; extraskeletal calcification; obesity;allergy, asthama; multiple sclerosis; muscle weakness; rheumatoidarthritis and diabetes.

In some embodiments, the treatment can comprise administering a compoundselected from the group consisting of: calcitriol; dihydrotachysterol;doxercalciferol; paricalcitol; cholecalciferol and ergocalciferol.

In another aspect, the invention relates to a system for obtaining datafrom at least one blood sample obtained from at least one subject, thesystem comprising: a determination module configured to receive the atleast one blood sample and perform at least one analysis on the at leastone blood sample to determine a level of bioavailable or free vitamin Din the sample; a storage device configured to store data output fromsaid determination module; and a display module for displaying a contentbased in part on the data output from said determination module, whereinthe content comprises a signal indicative of the level of bioavailableor free vitamin D.

In some embodiments, the system further comprises a means of inputting avalue for the level of one or more of VDBP polypeptide, albuminpolypeptide, and total vitamin D determined to be in a test sample. Insome embodiments, the content displayed on said display module furthercomprises a signal indicative of the subject having an increasedlikelihood of a vitamin D insufficiency if the level of bioavailable orfree vitamin D is determined to be lower than the 25^(th) percentilevalue, or than 25% of the mean value, of bioavailable vitamin D in apopulation of healthy subjects. In some embodiments, the contentdisplayed on said display module further comprises a signal indicativeof the subject being recommended to receive a treatment for vitamin Dinsufficiency.

In some embodiments, a level of free and/or bioavailable vitamin D lowerthan the 25^(th) percentile, or than 25% of the mean value, of freeand/or bioavailable vitamin D in a population of healthy subjects canindicate that the subject has a vitamin D insufficiency. In someembodiments, the vitamin D can be selected from the group consisting of:25-hydroxyvitamin D and 1,25-dihydroxyvitamin D.

In some embodiments, determining the level of VDBP polypeptide oralbumin polypeptide can comprise the use of a method selected from thegroup consisting of: enzyme linked immunosorbent assay; chemiluminescentimmunosorbent assay; electrochemiluminescent immunosorbent assay;fluorescent immunosorbent assay; dye linked immunosorbent assay;immunoturbidimetric assay; immunonephelometric assay; dye-basedphotometric assay; western blot; immunoprecipitation; radioimmunologicalassay (RIA); radioimmunometric assay; immunofluorescence assay and massspectroscopy.

In some embodiments, determining the level of total vitamin D cancomprise the use of a method selected from the group consisting of:radioimmunoassay; liquid chromatography tandem mass spectroscopy; enzymelinked immunosorbent assay; chemiluminescent immunosorbent assay;electrochemiluminescent immunosorbent assay; fluorescent immunosorbentassay; and high-pressure liquid chromatography.

In some embodiments, an insufficiency of vitamin D can indicate anincreased risk of a condition selected from the group consisting of:decreased bone density; decreased bone mineral density; bone fractures;bone resorption; rickets; osteitis fibrosa cystica; fibrogenesisimperfect ossium; osteosclerosis; osteoporosis; osteomalacia; elevatedparathyroid hormone levels; parathyroid gland hyperplasia; secondaryhyperparathyroidism; hypocalcemia; infection; cancer; psoriasis;cardiovascular disease; renal osteodystrophy; renal disease; end-stagerenal disease; chronic kidney disease; chronic kidney disease-associatedmineral and bone disorder; extraskeletal calcification; obesity;allergy, asthama; multiple sclerosis; muscle weakness; rheumatoidarthritis and diabetes.

In some embodiments, the invention can further comprise the step ofadministering a vitamin D insufficiency treatment to a subject who isdetermined to have a vitamin D insufficiency. In some embodiments, thetreatment can comprise administering a compound selected from the groupconsisting of: calcitriol; dihydrotachysterol; doxercalciferol;paricalcitol; cholecalciferol and ergocalciferol.

In another aspect, the invention relates to a method of treatmentcomprising: analyzing a blood sample obtained from a subject todetermine a level of free or bioavailable vitamin D; wherein a level offree or bioavailable vitamin D lower than the 25^(th) percentile, orthan 25% of the mean value of free or bioavailable vitamin D in apopulation of healthy subjects indicates that the subject has a vitaminD insufficiency; and administering a vitamin D insufficiency treatmentto a subject who is determined to have a vitamin D insufficiency.

In some embodiments, the vitamin D can be selected from the groupconsisting of: 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D.

In some embodiments, determining of the level of free and/orbioavailable vitamin D can comprise the use of a method selected fromthe group consisting of: immunoassay; two-step immnunoassay withantibody capture; one-step immunoassay with immobilized antibody andcompetitive detection; one-step immunoassay with immobilized competitorand labeled antibody; fluorescence polarization immunoassay;differential precipitation (immunoprecipitation, affinityprecipitation); fluorescence polarization VDBP binding assay;immunodepletion; and affinity binding chromatography; and a methodselected from the group consisting of: radioimmunoassay;chemiluminescent immunosorbent assay; electrochemiluminescentimmunosorbent assay; fluorescent immunosorbent assay; dye linkedimmunosorbent assay; liquid chromatography tandem mass spectroscopy andhigh-pressure liquid chromatography.

In some embodiments of all of the methods described herein, determininga level of free and/or bioavailable vitamin D can comprise the use ofchemically labeled 25-hydrovitamin D3 or vitamin D analogs with VDBPbinding affinity. These chemical modifications to incorporate labelingmoieties may first involve succinylation of the 3-hydroxyl group toyield 25-hydroxyvitamin D-3-hemisuccinate (Bouillon et al., Clin Chem.1984 November; 30(11):1731-6). This succinylation modification providesa linker group ending with a carboxyl moiety that can be furtherderivatized with carboxyl-reactive compounds that are chromatophoric,fluorescent, luminescent, chemiluminescent, electroluminescent, orlinked to enzymes. The labeling reaction of the carboxyl group on25-hydroxyvitamin D-3-hemisuccinate can be modified directly, e.g., withlabeling moieties containing diazoalkanes or alkyl halides; they canalso be labeled by hydrazines, hydroxylamines, or primary amines in thepresence of carbodiimide.

In some embodiments, an insufficiency of vitamin D can indicate anincreased risk of a condition selected from the group consisting of:decreased bone density; decreased bone mineral density; bone fractures;bone resorption; rickets; osteitis fibrosa cystica; fibrogenesisimperfect ossium; osteosclerosis; osteoporosis; osteomalacia; elevatedparathyroid hormone levels; parathyroid gland hyperplasia; secondaryhyperparathyroidism; hypocalcemia; infection; cancer; psoriasis;cardiovascular disease; renal osteodystrophy; renal disease; end-stagerenal disease; chronic kidney disease; chronic kidney disease-associatedmineral and bone disorder; extraskeletal calcification; obesity;allergy, asthama; multiple sclerosis; muscle weakness; rheumatoidarthritis and diabetes.

In some embodiments, the treatment can comprise administering a compoundselected from the group consisting of: calcitriol; dihydrotachysterol;doxercalciferol; paricalcitol; cholecalciferol and ergocalciferol.

In another aspect, the invention relates to an assay comprisinganalyzing a blood sample obtained from a subject to determine a level offree vitamin D and albumin polypeptide; wherein a level of bioavailablevitamin D is:

=(K _(alb)*[Alb]+1)*[Free Vitamin D].

In some embodiments, a level of bioavailable vitamin D lower than the25^(th) percentile, or than 25% of the mean value, of bioavailablevitamin D in a population of healthy subjects can indicate that thesubject has a vitamin D insufficiency. In some embodiments, the vitaminD can be selected from the group consisting of: 25-hydroxyvitamin D and1,25-dihydroxyvitamin D.

In some embodiments, determining the level of albumin polypeptide cancomprise the use of a method selected from the group consisting of:enzyme linked immunosorbent assay; chemiluminescent immunosorbent assay;electrochemiluminescent immunosorbent assay; fluorescent immunosorbentassay; dye linked immunosorbent assay; immunoturbidimetric assay;immunonephelometric assay; dye-based photometric assay; western blot;immunoprecipitation; radioimmunological assay (RIA); radioimmunometricassay; immunofluorescence assay and mass spectroscopy.

In some embodiments, determining of the level of free and/orbioavailable vitamin D can comprise the use of a method selected fromthe group consisting of: immunoassay; two-step immnunoassay withantibody capture; one-step immunoassay with immobilized antibody andcompetitive detection; one-step immunoassay with immobilized competitorand labeled antibody; fluorescence polarization immunoassay;differential precipitation (immunoprecipitation, affinityprecipitation); immunodepletion; and affinity binding chromatography;and a method selected from the group consisting of: radioimmunoassay;chemiluminescent immunosorbent assay; electrochemiluminescentimmunosorbent assay; fluorescent immunosorbent assay; dye linkedimmunosorbent assay; liquid chromatography tandem mass spectroscopy andhigh-pressure liquid chromatography.

Also provided herein are methods for treating a Vitamin D insufficiencyin a subject. The methods include determining a level of bioavailableVitamin D in the subject, by directly detecting levels of free Vitamin Dand Vitamin D bound to albumin in a sample comprising serum or plasmafrom the subject using a differential affinity precipitation assay;comparing the level of bioavailable Vitamin D in the sample to areference level of Vitamin D; identifying a subject who has a level ofbioavailable Vitamin D below the reference level of Vitamin D as havinga Vitamin D insufficiency; and administering a vitamin D insufficiencytreatment to a subject identified as having a vitamin D insufficiency.In some embodiments, a Vitamin D Index is calculated in the subject andcompared to a reference Index, below which a subject is identified ashaving a Vitamin D insufficiency, and a vitamin D insufficiencytreatment is administered to a subject who has a Vitamin D Index belowthe reference Index.

In some embodiments, the differential affinity precipitation assay isperformed by a method comprising contacting a sample comprising serum orplasma from the subject with purified Vitamin D Binding Polypeptide(VDBP), wherein the purified VDBP is immobilized on a substrate (e.g.,beads, solid surface) for a time sufficient for free and albumin-boundVitamin D in the sample to bind to the purified VDBP, thereby forming atest sample Vitamin D-VDBP complexes; optionally removing any Vitamin Dnot bound to the purified VDBP from the test sample; contacting theVitamin D-VDBP complexes with a known amount of free labeled Vitamin D,for a time sufficient for the labeled Vitamin D to equilibrate with theVitamin D-VDBP complexes in the test sample; determining the amount oflabeled Vitamin D bound to the purified VDBP in the test sample, andcalculating the amount of bioavailable Vitamin D in the sample from thesubject based on the amount of labeled Vitamin D bound to the purifiedVDBP in the test sample.

The VDBP can be immobilized on a substrate using any method known in theart. In some embodiments, the VDBP can be immobilized on a substratecoated with gold, biotin streptavidin, N-hydroxysuccinimide or oxiraneor other amine-reactive linker molecules, hydrazine or hydroxylamine orother carboxyl-reactive linker molecules, maleimide or otherthiol-reactive linker molecules, or another molecule suitable forimmobilizing the VDBP. In some embodiments, the VDBP is a fusion proteinand comprises a peptide immobilization moiety, e.g., Argtag,calmodulin-binding peptide, cellulose-binding domain, DsbA, c-myc-tag,glutathione S-transferase, FLAG-tag, HAT-tag, polyhistidine-tag,maltose-binding protein, NusA, Stag, SBP-tag, Strep-tag, MBP, SUMO,Protein A, Protein G, and/or thioredoxin, and the substrate is coatedwith the partner for the immobilization moiety; see, e.g., Terpe, ApplMicrobiol Biotechnol (2003) 60:523-533. In some embodiments, the VDBPare immobilized on the substrate via direct adsorption, peptide linkers,disulfide, maleimide, vinyl sulfone, or haloacetyl chemistries. Forexample, N- or C-terminal cysteine residues can be added to the VDBPusing standard techniques of molecular biology, allowing the modifiedVDBP to be attached directly to gold surfaces, or indirectly to suitablymodified glass or plastic surfaces, e.g., surfaces modified byintroduction of disulfide, maleimide, vinyl sulfone, and/or haloacetylchemistries. An azido group can be added, e.g., using the method ofexpressed protein ligation and a synthetic bifunctional reagent. Thisazido protein can be immobilized by Staudinger ligation. In someembodiments, the VDBP is covalently bound to the surface.

The beads or solid substrate can be any known in the art, includingmicroarrays (e.g., as described herein), polymeric beads (e.g.,polystyrene, acrylamide, acrylamide-azlactone), agarose beads (e.g.,activated agarose that allows direct binding of the VDBP to the bead,e.g., tosyl activated agarose), paramagnetic beads, multiwall plates, orother surfaces. In some embodiments the surface is a magnetic orparamagnetic bead. In some embodiments the surface is a porous gelsupport, e.g., sugar- or acrylamide-based polymer resins or beads.

In some embodiments, an insufficiency of vitamin D can indicate anincreased risk of a condition selected from the group consisting of:decreased bone density; decreased bone mineral density; bone fractures;bone resorption; rickets; osteitis fibrosa cystica; fibrogenesisimperfect ossium; osteosclerosis; osteoporosis; osteomalacia; elevatedparathyroid hormone levels; parathyroid gland hyperplasia; secondaryhyperparathyroidism; hypocalcemia; infection; cancer; psoriasis;cardiovascular disease; renal osteodystrophy; renal disease; end-stagerenal disease; chronic kidney disease; chronic kidney disease-associatedmineral and bone disorder; extraskeletal calcification; obesity;allergy, asthama; multiple sclerosis; muscle weakness; rheumatoidarthritis and diabetes.

In some embodiments, the invention can further comprise the step ofadministering a vitamin D insufficiency treatment to a subject who isdetermined to have a vitamin D insufficiency. In some embodiments, thetreatment can comprise administering a compound selected from the groupconsisting of: calcitriol; dihydrotachysterol; doxercalciferol;paricalcitol; cholecalciferol and ergocalciferol.

In some embodiments, the invention can further comprise the measurementof total serum or plasma 25-hydroxyvitamin D combined with analysis ofpatients' vitamin D binding protein variant genotypes as describedherein in order to interpret total 25-hydroxyvitamin D levels usinggenotype-specific reference levels, e.g., thresholds or intervals.Analysis of patients' VDBP variant genotype may be achieved by analyzingfor the presence of the Gc1F, Gc1S, or Gc2 protein variants usingchromatography, mass spectrometry, antibodies directed against thespecific variants, or by genotyping patients' DNA at their GC locus.

Thus, in another aspect, the invention includes methods for diagnosingand optionally treating a vitamin D insufficiency in a subject. Themethods include determining a VDBP genotype in the subject, e.g., bydetermining the identity of both alleles of one or both of the SNPslisted in Table A in a sample from the subject; determining a level oftotal vitamin D, and/or a level of free and/or bioavailable vitamin D,in a sample from the subject; and comparing the level of total, free,and/or bioavailable vitamin D to a reference level for the subject'sgenotype; wherein the presence of a level of total, free, and/orbioavailable vitamin D below the reference level indicates that thesubject has a vitamin D insufficiency. In some embodiments, the methodsfurther include administering a treatment for vitamin D insufficiency asknown in the art or described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the relationship between total and free 25-hydroxyvitaminD and lumbar spine bone mineral density. DBP-Bound, Free, andBioavailable 25-hydroxyvitamin D (25(OH)D) levels and the Vitamin DIndex were calculated from measured total 25(OH)D and vitamin D bindingprotein (DBP) levels. Total 25(OH)D and DBP-bound 25(OH)D were notassociated with lumbar spine bone mineral density (BMD). Free 25(OH)D,bioavailable 25(OH) D, and Vitamin D Index values were positivelycorrelated with lumbar spine BMD.

FIG. 2 depicts the relationship between total or bioavailable 25(OH)Dand serum calcium. Total levels of 25(OH)D demonstrated no associationwith serum calcium levels (corrected for albumin) while bioavailable25(OH)D levels were positively associated with serum calcium.

FIG. 3 depicts the relationship between total or bioavailable 25(OH)Dand PTH. After adjustment for age, gender, race, and survival status atone year, bioavailable 25(OH)D was significantly negatively associatedwith PTH levels, while total 25(OH)D demonstrated no association withPTH.

FIG. 4 depicts sample selection for Example 2. 25(OH)D and 1,25(OH)2Dwere previously measured as part of a case-control study within theArMORR cohort. Equal numbers of cases (subjects who died within theirfirst year on dialysis) and controls were randomly selected from eachracial group.

FIG. 5 depicts hypotheses tested in Example 4.

FIG. 6 is a diagram of an embodiment of a system for performing an assayfor determining the level of bioavailable or free vitamin D in a bloodsample obtained from a subject.

FIG. 7 is a diagram of an embodiment of a comparison module as describedherein.

FIG. 8 is a diagram of an embodiment of an operating system andapplications for a computing system as described herein.

FIG. 9A is a schematic showing variant VDBP proteins among black andwhite homozygotes.

FIG. 9B is a bar graph showing bioavailable 25-hydroxyvitamin D levelsin black and white homozygotes.

FIG. 9C is a schematic showing Vitamin D Binding Protein Variants resultin similar bioavailable 25(OH)D levels in blacks and whites.

FIG. 10 is a graph showing the percent bioavailable 25(OH)D radioligandmeasurements in VDBP calibrators.

FIG. 11 is a graph showing Vitamin D radioligand competitive bindingassay standard curve for conversion of radioligand binding measurementsinto equivalent calculated bioavailable 25(OH)D values.

FIG. 12 is a graph showing correlations between calculated bioavailable25(OH)D concentrations in subjects homozygous for VDBP protein variantsGc1F, Gc1S, and Gc2, compared to measurements by radioligand competitivebinding assay.

FIG. 13 is a schematic illustration of an exemplary differentialaffinity precipitation assay for bioavailable 25-hydroxyvitamin D.

FIG. 14 is a schematic illustration of an exemplary fluorescencepolarization assay for bioavailable and VDBP-bound 25-hydroxyvitamin D.

FIG. 15 is a graph showing measurements of bioavailable25-hydroxyvitamin D by affinity binding chromatography using immobilizedpurified DBP to extract bioavailable 25(OH)D from a standard solutionand from human serum. The data shows that increasing amounts of solubleDBP in samples decreases the amounts of affinity-extracted bioavailable25(OH)D.

DETAILED DESCRIPTION OF THE INVENTION

Aspects of the invention described herein include assays. Systems, andmethods of treatment which are based on the inventors' discovery thattotal serum levels of vitamin D, the currently used clinical parameter,correlate poorly with measures of health such as bone mineral densityand parathyroid hormone levels. The inventors have found that levels ofbioavailable and free vitamin D correlate better with the same measuresof health and are therefore more indicative of whether a subject hassufficient vitamin D levels. Described herein are assays for measuringbioavailable and free vitamin D and methods of treating subjects forvitamin D insufficiency.

Materials, procedures and considerations necessary to understand and usethe disclosed methods are described in the following, as areexperimental results and non-limiting examples that demonstrate andillustrate various embodiments of the methods and assays described.

DEFINITIONS

For convenience, certain terms employed herein, in the specification,examples and appended claims are collected here. Unless statedotherwise, or implicit from context, the following terms and phrasesinclude the meanings provided below. Unless explicitly stated otherwise,or apparent from context, the terms and phrases below do not exclude themeaning that the term or phrase has acquired in the art to which itpertains. The definitions are provided to aid in describing particularembodiments, and are not intended to limit the claimed invention,because the scope of the invention is limited only by the claims. Unlessotherwise defined, all technical and scientific terms used herein havethe same meaning as commonly understood by one of ordinary skill in theart to which this invention belongs.

As used herein the term “comprising” or “comprises” is used in referenceto compositions, methods, and respective component(s) thereof, that areessential to the method or composition, yet open to the inclusion ofunspecified elements, whether essential or not.

As used herein the term “consisting essentially of” refers to thoseelements required for a given embodiment. The term permits the presenceof elements that do not materially affect the basic and novel orfunctional characteristic(s) of that embodiment.

The term “consisting of” refers to compositions, methods, and respectivecomponents thereof as described herein, which are exclusive of anyelement not recited in that description of the embodiment.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural references unless the contextclearly dictates otherwise. Thus for example, references to “the method”includes one or more methods, and/or steps of the type described hereinand/or which will become apparent to those persons skilled in the artupon reading this disclosure and so forth. Similarly, the word “or” isintended to include “and” unless the context clearly indicatesotherwise. Although methods and materials similar or equivalent to thosedescribed herein can be used in the practice or testing of thisdisclosure, suitable methods and materials are described below. Theabbreviation, “e.g.” is derived from the Latin exempli gratia, and isused herein to indicate a non-limiting example. Thus, the abbreviation“e.g.” is synonymous with the term “for example.”

Definitions of common terms in cell biology and molecular biology can befound in “The Merck Manual of Diagnosis and Therapy”, 19th Edition,published by Merck Research Laboratories, 2006 (ISBN 0-911910-19-0);Robert S. Porter et al. (eds.), The Encyclopedia of Molecular Biology,published by Blackwell Science Ltd., 1994 (ISBN 0-632-02182-9); TheELISA guidebook (Methods in molecular biology 149) by Crowther J. R.(2000); Fundamentals of RIA and Other Ligand Assays by Jeffrey Travis,1979, Scientific Newsletters; Immunology by Werner Luttmann, publishedby Elsevier, 2006. Definitions of common terms in molecular biology canalso be found in Benjamin Lewin, Genes X, published by Jones & BartlettPublishing, 2009 (ISBN-10: 0763766321); Kendrew et al. (eds.).,Molecular Biology and Biotechnology: a Comprehensive Desk Reference,published by VCH Publishers, Inc., 1995 (ISBN 1-56081-569-8) and CurrentProtocols in Protein Sciences 2009, Wiley Intersciences, Coligan et al.,eds.

The terms “decrease,” “reduce,” “reduced”, “reduction”, “decrease,” and“inhibit” are all used herein generally to mean a decrease by astatistically significant amount relative to a reference. However, foravoidance of doubt, “reduce,” “reduction” or “decrease” or “inhibit”typically means a decrease by at least 10% as compared to a referencelevel and can include, for example, a decrease by at least about 20%, atleast about 25%, at least about 30%, at least about 35%, at least about40%, at least about 45%, at least about 50%, at least about 55%, atleast about 60%, at least about 65%, at least about 70%, at least about75%, at least about 80%, at least about 85%, at least about 90%, atleast about 95%, at least about 98%, at least about 99%, up to andincluding, for example, the complete absence of the given entity orparameter as compared to a reference level, or any decrease between10-99% as compared to a reference level.

The terms “increased”, “increase” or “enhance” or “activate” or“promote” are all used herein to generally mean an increase by astatically significant amount: for the avoidance of any doubt, the terms“increased”, “increase” or “enhance” or “activate” or “promote” means anincrease of at least 10% as compared to a reference level, for examplean increase of at least about 20%, or at least about 30%, or at leastabout 40%, or at least about 50%, or at least about 60%, or at leastabout 70%, or at least about 80%, or at least about 90% or up to andincluding a 100% increase or any increase between 10-100% as compared toa reference level, or at least about a 2-fold, or at least about a3-fold, or at least about a 4-fold, or at least about a 5-fold or atleast about a 10-fold increase, or any increase between 2-fold and10-fold or greater as compared to a reference level.

As used herein, the term “proteins” and “polypeptides” are usedinterchangeably herein to designate a series of amino acid residuesconnected to the other by peptide bonds between the alpha-amino andcarboxy groups of adjacent residues. The terms “protein”, and“polypeptide”, which are used interchangeably herein, refer to a polymerof protein amino acids, including modified amino acids (e.g.,phosphorylated, glycated, glycosylated, etc.) and amino acid analogs,regardless of its size or function. “Protein” and “polypeptide” areoften used in reference to relatively large polypeptides, whereas theterm “peptide” is often used in reference to small polypeptides, butusage of these terms in the art overlaps. The terms “protein” and“polypeptide” are used interchangeably herein when referring to a geneproduct and fragments thereof. Thus, exemplary polypeptides or proteinsinclude gene products, naturally occurring proteins, homologs,orthologs, paralogs, fragments and other equivalents, variants,fragments, and analogs of the foregoing.

As used herein, an “allele” is one of a pair or series of geneticvariants of a polymorphism at a specific genomic location.

As used herein, “genotype” refers to the diploid combination of allelesfor a given genetic polymorphism. A homozygous subject carries twocopies of the same allele and a heterozygous subject carries twodifferent alleles.

As used herein, a “haplotype” is one or a set of signature geneticchanges (polymorphisms) that are normally grouped closely together onthe DNA strand, and are usually inherited as a group; the polymorphismsare also referred to herein as “markers.” A “haplotype” as used hereinis information regarding the presence or absence of one or more geneticmarkers in a given chromosomal region in a subject. A haplotype canconsist of a variety of genetic markers, including indels (insertions ordeletions of the DNA at particular locations on the chromosome); singlenucleotide polymorphisms (SNPs) in which a particular nucleotide ischanged; microsatellites; and minisatellites.

The term “gene” refers to a DNA sequence in a chromosome that codes fora product (either RNA or its translation product, a polypeptide). A genecontains a coding region and includes regions preceding and followingthe coding region (termed respectively “leader” and “trailer”). Thecoding region is comprised of a plurality of coding segments (“exons”)and intervening sequences (“introns”) between individual codingsegments.

The term “probe” refers to an oligonucleotide. A probe can be singlestranded at the time of hybridization to a target. As used herein,probes include primers, i.e., oligonucleotides that can be used to primea reaction, e.g., a PCR reaction.

The term “label” or “label containing moiety” refers in a moiety capableof detection, such as a radioactive isotope or group containing same,and nonisotopic labels, such as enzymes, biotin, avidin, streptavidin,digoxygenin, luminescent agents, dyes, haptens, and the like.Luminescent agents, depending upon the source of exciting energy, can beclassified as radioluminescent, chemiluminescent, bioluminescent, andphotoluminescent (including fluorescent and phosphorescent). A probedescribed herein can be bound, e.g., chemically bound tolabel-containing moieties or can be suitable to be so bound. The probecan be directly or indirectly labeled.

The term “direct label probe” (or “directly labeled probe”) refers to anucleic acid probe whose label after hybrid formation with a target isdetectable without further reactive processing of hybrid. The term“indirect label probe” (or “indirectly labeled probe”) refers to anucleic acid probe whose label after hybrid formation with a target isfurther reacted in subsequent processing with one or more reagents toassociate therewith one or more moieties that finally result in adetectable entity.

The terms “target,” “DNA target,” or “DNA target region” refers to anucleotide sequence that occurs at a specific chromosomal location. Eachsuch sequence or portion is preferably at least partially, singlestranded (e.g., denatured) at the time of hybridization. When the targetnucleotide sequences are located only in a single region or fraction ofa given chromosome, the term “target region” is sometimes used. Targetsfor hybridization can be derived from specimens which include, but arenot limited to, chromosomes or regions of chromosomes in normal,diseased or malignant human cells, either interphase or at any state ofmeiosis or mitosis, and either extracted or derived from living orpostmortem tissues, organs or fluids; germinal cells including sperm andegg cells, or cells from zygotes, fetuses, or embryos, or chorionic oramniotic cells, or cells from any other germinating body; cells grown invitro, from either long-term or short-term culture, and either normal,immortalized or transformed; inter- or intraspecific hybrids ofdifferent types of cells or differentiation states of these cells;individual chromosomes or portions of chromosomes, or translocated,deleted or other damaged chromosomes, isolated by any of a number ofmeans known to those with skill in the art, including libraries of suchchromosomes cloned and propagated in prokaryotic or other cloningvectors, or amplified in vitro by means well known to those with skill;or any forensic material, including but not limited to blood, or othersamples.

The term “hybrid” refers to the product of a hybridization procedurebetween a probe and a target.

The term “hybridizing conditions” has general reference to thecombinations of conditions that are employable in a given hybridizationprocedure to produce hybrids, such conditions typically involvingcontrolled temperature, liquid phase, and contact between a probe (orprobe composition) and a target. Conveniently and preferably, at leastone denaturation step precedes a step wherein a probe or probecomposition is contacted with a target. Guidance for performinghybridization reactions can be found in Ausubel et al., CurrentProtocols in Molecular Biology, John Wiley & Sons, N.Y. (2003),6.3.1-6.3.6. Aqueous and nonaqueous methods are described in thatreference and either can be used. Hybridization conditions referred toherein are a 50% formamide, 2×SSC wash for 10 minutes at 45° C. followedby a 2×SSC wash for 10 minutes at 37° C.

Calculations of “identity” between two sequences can be performed asfollows. The sequences are aligned for optimal comparison purposes(e.g., gaps can be introduced in one or both of a first and a secondnucleic acid sequence for optimal alignment and non-identical sequencescan be disregarded for comparison purposes). The length of a sequencealigned for comparison purposes is at least 30% (e.g., at least 40%,50%, 60%, 70%, 80%, 90% or 100%) of the length of the referencesequence. The nucleotides at corresponding nucleotide positions are thencompared. When a position in the first sequence is occupied by the samenucleotide as the corresponding position in the second sequence, thenthe molecules are identical at that position. The percent identitybetween the two sequences is a function of the number of identicalpositions shared by the sequences, taking into account the number ofgaps, and the length of each gap, which need to be introduced foroptimal alignment of the two sequences.

The comparison of sequences and determination of percent identitybetween two sequences can be accomplished using a mathematicalalgorithm. In some embodiments, the percent identity between twonucleotide sequences is determined using the GAP program in the GCGsoftware package, using a Blossum 62 scoring matrix with a gap penaltyof 12, a gap extend penalty of 4, and a frameshift gap penalty of 5.

The term “nonspecific binding DNA” refers to DNA which is complementaryto DNA segments of a probe, which DNA occurs in at least one otherposition in a genome, outside of a selected chromosomal target regionwithin that genome. An example of nonspecific binding DNA comprises aclass of DNA repeated segments whose members commonly occur in more thanone chromosome or chromosome region. Such common repetitive segmentstend to hybridize to a greater extent than other DNA segments that arepresent in probe composition.

As used herein, “determining the identity of an allele” includesobtaining information regarding the identity, presence or absence of oneor more specific alleles in a subject. Determining the identity of anallele can, but need not, include obtaining a sample comprising DNA froma subject, and/or assessing the identity, presence or absence of one ormore genetic markers in the sample. The individual or organization whodetermines the identity of the allele need not actually carry out thephysical analysis of a sample from a subject; the methods can includeusing information obtained by analysis of the sample by a third party.Thus the methods can include steps that occur at more than one site. Forexample, a sample can be obtained from a subject at a first site, suchas at a health care provider, or at the subject's home in the case of aself-testing kit. The sample can be analyzed at the same or a secondsite, e.g., at a laboratory or other testing facility.

In some embodiments, to determine the identity of an allele orpresence/absence of an allele or genotype described herein, a biologicalsample that includes nucleated cells (such as blood, a cheek swab ormouthwash) is prepared and analyzed for the presence or absence ofpreselected markers. Such diagnoses may be performed by diagnosticlaboratories, or, alternatively, diagnostic kits can be manufactured andsold to health care providers or to private individuals forself-diagnosis. Diagnostic or prognostic tests can be performed asdescribed herein or using well known techniques, such as described inU.S. Pat. No. 5,800,998.

Results of these tests, and optionally interpretive information, can bereturned to the subject, the health care provider or to a third partypayor. The results can be used in a number of ways. The information canbe, e.g., communicated to the tested subject, e.g., with a prognosis andoptionally interpretive materials that help the subject understand thetest results and prognosis. The information can be used, e.g., by ahealth care provider, to determine whether to administer a specificdrug, or whether a subject should be assigned to a specific category.e.g., a category associated with a specific disease endophenotype, orwith drug response or non-response. The information can be used, e.g.,by a third party payor such as a healthcare payer (e.g., insurancecompany or HMO) or other agency, to determine whether or not toreimburse a health care provider for services to the subject, or whetherto approve the provision of services to the subject. For example, thehealthcare payer may decide to reimburse a health care provider fortreatments for vitamin D deficiency if the subject has vitamin Ddeficiency. The presence or absence of the allele or genotype in apatient may be ascertained by using any of the methods described herein.

As used herein, the term “pharmaceutical composition” refers to theactive agent in combination with a pharmaceutically acceptable carrieras commonly used in the pharmaceutical industry.

The phrase “pharmaceutically acceptable” is employed herein to refer tothose compounds, materials, compositions, and/or dosage forms which are,within the scope of sound medical judgment, suitable for use in contactwith the tissues of human beings and animals without excessive toxicity,irritation, allergic response, or other problem or complication,commensurate with a reasonable benefit/risk ratio.

The phrase “pharmaceutically acceptable carrier” includes any and allsolvents, dispersion media, coatings, antibacterial and antifungalagents, isotonic and absorption delaying agents and the like. The use ofsuch media and agents for pharmaceutical active substances is well knownin the art. Except insofar as any conventional media or agent isincompatible with the active ingredient or is toxic to the subject, usethereof in the therapeutic compositions is contemplated. Supplementaryactive ingredients can also be incorporated into the compositions.

As used herein, a “subject” means a human or animal. Usually the animalis a vertebrate such as a primate, rodent, domestic animal or gameanimal. Primates include chimpanzees, cynomologous monkeys, spidermonkeys, and macaques, e.g., Rhesus. Rodents include mice, rats,woodchucks, ferrets, rabbits and hamsters. Domestic and game animalsinclude cows, horses, pigs, deer, bison, buffalo, feline species, e.g.,domestic cat, canine species, e.g., dog, fox, wolf, avian species, e.g.,chicken, emu, ostrich, and fish, e.g., trout, catfish and salmon.Patient or subject includes any subset of the foregoing, e.g., all ofthe above. In certain embodiments, the subject is a mammal, e.g., aprimate, e.g., a human.

Preferably, the subject is a mammal. The mammal can be a human,non-human primate, mouse, rat, dog, cat, horse, or cow, but is notlimited to these examples. Mammals other than humans can beadvantageously used as subjects that represent animal models of vitaminD insufficiency. In addition, the methods described herein can be usedto treat domesticated animals and/or pets. A subject can be male orfemale. A subject can be one who has been previously diagnosed with oridentified as suffering from or having vitamin D insufficiency or one ormore diseases or conditions associated with a vitamin D insufficiency,and optionally, but need not have already undergone treatment forvitamin D insufficiency or the one or more diseases or conditionsassociated with a vitamin D insufficiency. A subject can also be one whohas been diagnosed with or identified as suffering from vitamin Dinsufficiency or one or more diseases or conditions associated with avitamin D insufficiency, but who shows improvements in known vitamin Dinsufficiency risk factors as a result of receiving one or moretreatments for vitamin D insufficiency or one or more diseases orconditions associated with a vitamin D insufficiency. Alternatively, asubject can also be one who has not been previously diagnosed as havingvitamin D insufficiency or one or more diseases or conditions associatedwith a vitamin D insufficiency. For example, a subject can be one whoexhibits one or more risk factors for vitamin insufficiency or one ormore diseases or conditions associated with a vitamin D insufficiency ora subject who does not exhibit vitamin D insufficiency risk factors.

Other than in the operating examples, or where otherwise indicated, allnumbers expressing quantities of ingredients or reaction conditions usedherein should be understood as modified in all instances by the term“about.” The term “about” when used in connection with percentages canmean±1%.

The term “statistically significant” or “significantly” refers tostatistical significance and generally means a difference of at leasttwo standard deviations (2SD).

Vitamin D Insufficiency

Aspects of the invention described herein include assays directed todetermining whether a subject has a vitamin D insufficiency and methodsof treating these conditions. As used herein, “vitamin D” refers to anyof several forms of D vitamins including vitamin D1, D2, D3, D4 andisomers or derivatives thereof. Non-limiting examples of forms ofvitamin D include vitamin D2 (ergocalciferol) which is produced byplants, vitamin D3 (cholecalciferol) which is produced by animals. Bothvitamin D2 and D3 are hydroxylated in the liver to form, respectively,25(OH)D2 (25-hydroxyvitamin D2) and 25(OH)D3 (25-hydroxyvitamin D3 orcalcidiol), which can be referred to collectively as 25(OH)D. 25(OH)D isthe primary transport form of vitamin D in the body and is theprohormone of the active vitamin D hormones. Further hydroxylation,primarily in the kidneys, converts 25(OH)D to 1,25(OH)₂D (including1,25(OH)₂D3 (calcitriol) and 1,25(OH)₂D2). All of the foregoing forms ofvitamin D are encompassed by the term “vitamin D” as used herein. Insome embodiments, the level of vitamin D that is determined is the levelof 25-(OH)D. In some embodiments, the level of vitamin D that isdetermined is the level of 25-(OH)D2. In some embodiments, the level ofvitamin D that is determined is the level of 25-(OH)D3. In someembodiments, the level of vitamin D that is determined is the level of1,25-(OH)₂D. In some embodiments, the level of vitamin D that isdetermined is the level of 1,25-(OH)₂D2. In some embodiments, the levelof vitamin D that is determined is the level of 1,25-(OH)₂D3.

Vitamin D hormones influence bone mineralization and a number of aspectsof blood chemistry, including blood calcium levels and blood phosphoruslevels. Diseases and conditions which are caused by or associated withinsufficient levels of vitamin D are referred to herein as “vitaminD-associated diseases.” Insufficient levels of vitamin D are associatedwith secondary hyperparathyroidism, parathyroid gland hyperplasia,elevated parathyroid hormone levels, hypocalcemia, chronic kidneydisease (CKD), renal disease, end-stage renal disease, chronic kidneydisease-associated mineral and bone disorder, psoriasis, low bonemineral density, bone resorption and metabolic bone diseases such asfibrogenesis imperfecta ossium, osteitis fibrosa cystica, osteomalacia,rickets, osteoporosis, osteosclerosis, non-traumatic fractures of thespine and hip, renal osteodystrophy, and extraskeletal calcification.Secondary hyperparathyroidism (SHPT) increases bone turnover, and ifleft untreated, can impair mineralization and decrease bone mass.Patients with SHPT have increased bone turnover and decreased bone massthat can eventually progress to osteomalacia. Osteomalacia is a severedefect in or absence of bone mineralization occurring when both vitaminD and dietary calcium levels are markedly reduced. Osteoporosis, definedas a deficiency of normal bone within bone tissue, can result eitherfrom a low calcium diet with replete vitamin D levels or with lowvitamin D and adequate dietary calcium. Low serum 25(OH)D increases therisk of osteoporotic fractures, especially in older adults, and vitaminD and calcium supplementation at sufficient doses reduces the risk. Anumber of “non-classical” biologic effects have been reported forvitamin D beyond its “classical” effects on the parathyroid hormonesystem. Such effects have been reported in connection with cellulargrowth and differentiation, cellular proliferation, red blood cellformation, hair growth, muscular function, blood pressure, fibrosis, theimmune system and the cardiovascular system, including therenin-angiotensin system. Vitamin D insufficiency has been implicated inthe development or progression of, for example, infection,cardiovascular disease, allergy, asthama obesity, diabetes, muscleweakness, multiple sclerosis, rheumatoid arthritis and cancer.

Vitamin D insufficiency can be caused by insufficient exposure tosunlight, insufficient dietary intake of vitamin D, or conditions orclinical procedures, such as bariatric surgery, that result in reducedintestinal absorption of fat soluble vitamins such as vitamin D. VitaminD levels are traditionally measured as the level of total serum 25(OH)D.Although total serum 25(OH)D is currently the most widely accepted assayfor determining vitamin D status, it is subject to tremendous variationin results and interpretation, and may not be clinically relevant acrossall populations. Aspects of the invention described herein are directedto an assay for determining whether a subject is vitamin D insufficientby measuring the levels of bioavailable or free vitamin D, as opposed tototal serum vitamin D.

As used herein, “vitamin D insufficiency” refers to suboptimal levels ofvitamin D that can be associated with an increased risk of developingone or more of the conditions or diseases in which low vitamin D levelshave been implicated, which are described above herein. Subjects with avitamin D insufficiency may not have any symptoms, markers or signs of avitamin D-associated disease or may have symptoms, markers or signs ofone or more vitamin D-associated diseases. A subject who has a vitamin Dinsufficiency can be a subject who has a level of bioavailable or freevitamin D which is lower than a threshold level, e.g., the 25thpercentile value or 25% or lower than the mean level, of that form ofvitamin D measured in a healthy population of subjects. For example, asubject who has a level of bioavailable or free vitamin D which is 25%,or 20% or 15% or 10% or 5% or lower than the mean level of that form ofvitamin D in a population of healthy subjects has an insufficient levelof vitamin D.

A healthy subject can be one who does not display any markers, signs orsymptoms of a vitamin D-associated disease or condition and who is notat risk of having a vitamin D insufficiency. By way of non-limitingexample, a healthy subject will not exhibit signs or symptoms ofrickets, which include, for example delayed growth, pain in the spine,pelvis or legs, muscle weakness, or skeletal deformities such as bowedlegs, abnormal curvature of the spine, thickened wrists and ankles,and/or projection of the breastbone. Risk factors for vitamin Dinsufficiency are well-known in the art and can include, but are notlimited to, not drinking vitamin D fortified milk (e.g. lactoseintolerant subjects, subjects with milk allergies, some vegetarians, andbreast-fed infants); dark skin; old age (e.g. the elderly have a reducedability to synthesize vitamin D and can be more likely to stay indoors),chronic or acute or severe illness (conditions which make it likely thesubject will stay indoors, in hospitals, in intensive care facilities,or institutional and assisted-care facilities, including subjects withAlzheimer's disease or who are mentally ill); covering all exposed skin(such as members of certain religions or cultures); regular use ofsunscreen (e.g., the application of sunscreen with a Sun ProtectionFactor (SPF) value of 8 reduces production of vitamin D by 95%, andhigher SPF values may further reduce vitamin D); having or having beendiagnosed with a fat malabsorption syndrome (including but not limitedto cystic fibrosis, cholestatic liver disease, other liver disease,gallbladder disease, pancreatic enzyme deficiency, Crohn's disease,inflammatory bowel disease, sprue or celiac disease, or surgical removalof part or all of the stomach and/or intestines); having had small bowelresections; taking medications that increase the catabolism of vitaminD, including phenytoin, fosphenytoin, phenobarbital, carbamazepine, andrifampin; taking medications that reduce absorption of vitamin D,including cholestyramine, colestipol, orlistat, mineral oil, and fatsubstitutes; taking medications that inhibit activation of vitamin D,including ketoconazole; taking medications that decrease calciumabsorption, including corticosteroids; having or having been diagnosedas having gum disease, diabetes mellitus, insulin resistance syndrome,endothelial dysfunction (vitamin D deposited in body fat stores is lessbioavailable) cardiovascular disease, artherosclerosis heart failure orosteoporosis; being obese; or being a postmenopausal woman.

Bioavailable Vitamin D

The assays and methods of treatments described herein are based on theinventors' discovery that the level of bioavailable and/or free vitaminD in the blood of a subject has a more significant correlation tomeasures of health such as bone mineral density and parathyroid hormonelevels than does the level of total serum vitamin D.

In the blood stream, vitamin D can exist in one of three states; 1)bound by vitamin D binding protein, 2) bound by albumin protein or 3)unbound. As used herein, “vitamin D binding protein”, “VDBP” or “DBP”refers to a polypeptide of any of SEQ ID NO: 1, 2 or 3 (NCBI Gene ID No:2638) and naturally occurring variants (e.g. alleles), homologs andfunctional derivatives thereof. VDBP binds vitamin D tightly, with aK_(D)=0.7×10⁹ M⁻¹ (for human VDBP). The fraction of vitamin D which isbound to VDBP is referred to herein as “D_(VDBP)”, “D_(DBP)”, “VitaminD_(DBP)” or “Vitamin D_(VDBP).” As used herein. “albumin” refers to thepolypeptide of any of SEQ ID NO: 4, 5 or 6 (NCBI Gene ID No: 213) andnaturally occurring variants (e.g. alleles), homologs and functionalderivatives thereof. Albumin binds vitamin D less tightly than VDBP,with a K_(D)=6×10⁵ M⁻¹ (for human albumin). The fraction of vitamin Dwhich is bound to albumin is referred to herein as “D_(albumin)”,“D_(Alb)”, “vitamin D_(albumin)” or “vitamin D_(Alb).” Unbound vitamin Dis also referred to herein as “free vitamin D” or “D.” As used herein,“bioavailable vitamin D” refers to, collectively, free vitamin D andvitamin D bound to albumin. Bioavailable vitamin D does not comprise thefraction of vitamin D which is bound to VDBP. Bioavailable vitamin D isinterchangeably referred to herein as “V_(Bio)” and “Vitamin D_(Bio)”.

An Assay for Bioavailable Vitamin D

Aspects of the invention described herein are directed to assays todetermine the level of bioavailable and/or free vitamin D in a bloodsample obtained from a subject.

In some embodiments, the level of bioavailable and/or free vitamin D isdetermined by first determining the level of VDBP polypeptide, albuminpolypeptide and total vitamin D in a blood sample obtained from asubject. The level of free and bioavailable vitamin D can then becalculated using these values and the binding constants of VDBP andalbumin for vitamin D. For human proteins, the binding constants are,respectively, 0.7×10⁹ M⁻¹ and 6×10⁵ M⁻¹.

As used herein a “blood sample” refers to any amount of blood or afraction thereof that has been obtained from a subject. In someembodiments, the blood sample can comprise whole blood or a fractionthereof, e.g. serum or plasma. In some embodiments, the blood sample iscontacted with an anticoagulant or preservative prior to performing anassay as described herein. Non-limiting examples of anticoagulants andpreservatives include CPD, CP2D (Citrate Phosphate Double Dextrose),CPDA-1, CDP/ADSOL®, CDP/Optisol®, AS-3 (Additive Solution 3, HaemoneticsCorp Braintree Mass.) and SAG-M.

In some embodiments, a blood sample can be stored prior to being used inan assay as described herein. In some embodiments the blood sample canbe stored for any given period of time, e.g. minutes, hours, days,weeks, up to months, prior to use in an assay as described herein. Inone embodiment, the blood sample is frozen. In one embodiment, the bloodsample is not frozen.

In some embodiments, the assays described herein are performed on awhole blood sample. In some embodiments, the assays described herein areperformed on the plasma fraction of a blood sample. In some embodiments,the assays described herein are performed on the serum fraction of ablood sample.

The level of VDBP and/or albumin polypeptide present in the blood sampleobtained from a subject can be determined by any method for determiningthe level of a specific polypeptide known in the art. In someembodiments, the assay is performed on an automated analyzer.Non-limiting examples of methods that can be used in the methods andassays described herein include enzyme linked immunosorbent assay;dye-based photometric assay; western blot; immunoprecipitation;radioimmunological assay (RIA); radioimmunometric assay;chemiluminescent immunosorbent assay; electrochemiluminescentimmunosorbent assay; fluorescent immunosorbent assay; dye linkedimmunosorbent assay; immunoturbidimetric assay; immunonephelometricassay; immunofluorescence assay and mass spectroscopy. Various methodsof producing antibodies with a known antigen (e.g. a peptide comprisedby the polypeptides of SEQ ID Nos. 1-6 are well-known to thoseordinarily skilled in the art (see Antibodies: A Laboratory Manual(Harlow & Lane eds., 1988), which is hereby incorporated by reference inits entirety). In particular, suitable antibodies may be produced bychemical synthesis, by intracellular immunization (i.e., intrabodytechnology), or preferably, by recombinant expression techniques.Methods of producing antibodies may further include the hybridomatechnology well-known in the art. Antibodies specific for albumin andVDBP are commercially available (e.g. Cat. # ab112888 and ab23484,respectively, Abcam; Cambridge, Mass.).

In some embodiments, albumin levels can be determined by dye-basedphotometric assays on an automated analyzer. Dye-based photometricassays are commercially available (e.g. the Albumin FS™ kits; DiaSysDiagnostic Systems Gmb; Holzheim, Germany or the Albumin reagent, Cat#OSR6102; Beckman Coulter; Brea, Calif.). Automated analyzers arecommercially available (e.g. the AU2700 or AU5400 from Beckman Coulter;Brea, Calif.). Systems which are designed specifically for thedetermination of serum albumin levels are also available commercially(e.g. the Careside Analyzer™, Careside Inc., Culver City, Calif.). Insome embodiments, the level of albumin levels can be determined usingimmunoassays, e.g. the Human Serum Albumin ELISA Kit (Cat #1190; AlphaDiagnostic International; San Antonio, Tex.).

In some embodiments, VDBP levels can be determined by ELISA. ELISAassays for VDBP are commercially available (e.g. Cat #DVDBP0; R&DSystems; Minneapolis, Minn.). In some embodiments, the assay isconducted after diluting serum samples 1 to 2,000 in Calibrator DiluentRD6-11 (R&D Systems Part Number 895489). ELISA is a technique fordetecting and measuring the concentration of an antigen using a labeled(e.g. enzyme linked) form of the antibody. There are different forms ofELISA, which are well known to those skilled in the art. The standardtechniques known in the art for ELISA are described in “Methods inImmunodiagnosis”, 2nd Edition, Rose and Bigazzi, eds. John Wiley & Sons,1980; Campbell et al., “Methods and Immunology”, W. A. Benjamin. Inc.,1964; and Oellerich, M. 1984, J. Clin. Chem. Clin. Biochem., 22:895-904;which are incorporated by reference herein in their entirety.

The level of total vitamin D present in the blood sample obtained from asubject can be determined by any method known in the art. Non-limitingexamples of methods that can be used in the methods and assays describedherein include radioimmunoassay; liquid chromatography tandem massspectroscopy; enzyme linked immunosorbent assay; chemiluminescentimmunosorbent assay; electrochemiluminescent immunosorbent assay;fluorescent immunosorbent assay; and high-pressure liquidchromatography. In some embodiments, the level of total vitamin D in asample is determined by liquid chromatography tandem mass spectrometry(LC-MS). In some embodiments, the level of total vitamin D in a sampleis determined by high performance liquid chromatography/massspectrophotometry. In some embodiments, the level of total vitamin D ina sample is determined by radioimmunoassay. In some embodiments, thelevel of total vitamin D in a sample is determined using a commerciallyavailable radioimmunoassay (e.g. DiaSorin Inc, Stillwater, Minn., USA).In some embodiments, the level of total vitamin D in a sample isdetermined using a commercially available immunluminometric assay (e.g.Cat No 310600; DiaSorin Inc.; Stillwater MIN). The method of measuringtotal vitamin D in a blood sample obtained from a subject can alsoinclude liquid chromatography tandem mass spectroscopy as described inU.S. Pat. No. 7,700,365, which is included by reference herein in itsentirety.

Mass spectroscopy methods are well known in the art and have been usedto quantify and/or identify biomolecules. In some embodiments, thesignal strength of peak values from spectra of a first sample and asecond sample can be compared (e.g., visually, by computer analysisetc.), to determine the relative amounts of particular biomolecules.Software programs such as the Biomarker Wizard program (CiphergenBiosystems, Inc., Fremont. Calif.) can be used to aid in analyzing massspectra.

In some embodiments, the level of total vitamin D which is determinedcan comprise one or more forms of vitamin D selected from the groupconsisting of 25-hydroxyvitamin D (25(OH)D); 1,25-dihydroxyvitamin D(1,25-(OH)₂D); 25(OH)D2; 25(OH)D3; 1,25(OH)₂D2; 1,25-(OH)D3; vitamin D1;vitamin D2; vitamin D3; vitamin D4; ergocalciferol; cholecalciferol;calcidiol and calcitriol. In some embodiments, the level of totalvitamin D which is determined can comprise 25-hydroxyvitamin D(25(OH)D). In some embodiments, the level of total vitamin D which isdetermined can comprise 25-hydroxyvitamin D2 (25(OH)D2). In someembodiments, the level of total vitamin D which is determined cancomprise 25-hydroxyvitamin D3 (25(OH)D3). In some embodiments, the levelof total vitamin D which is determined can comprise1,25-dihydroxyvitamin D (1,25-(OH)₂D). In some embodiments, the level oftotal vitamin D which is determined can comprise 1,25-dihydroxyvitaminD2 (1,25-(OH)₂D2). In some embodiments, the level of total vitamin Dwhich is determined can comprise 1,25-dihydroxyvitamin D3(1,25-(OH)₂D3).

In some embodiments, once the levels of VDBP polypeptide, albuminpolypeptide and total vitamin D in a blood sample obtained from asubject are determined, the level of free and/or bioavailable vitamin Dcan be determined. The level of free vitamin D can be calculated usingEquation 8, the derivation of which is described in Example 1 herein.

Free Vitamin D={−{K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1}+√{(K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1)²+4·(K _(DBP) ·K _(alb)·[Alb]+K_(DBP))·([Total VitaminD])}}÷(2·{K _(DBP) ·K _(alb)·[Alb]+K _(DBP)})  (Eq. 8)

The level of bioavailable vitamin D can be calculated using Equation 9,the derivation of which is described in Example 1 herein.

Bioavailable Vitamin D=(K _(alb)·[Alb]+1)·[Free Vitamin D]  (Eq. 9)

In some embodiments, the level of free (unbound) and/or bioavailablevitamin D can be determined directly. In some embodiments, the level offree vitamin D can be determined directly and used to calculate thelevel of bioavailable vitamin D. In some embodiments, the level of freevitamin D and the level of albumin polypeptide can be determineddirectly and used to calculate the level of bioavailable vitamin D. Asused herein “determined directly” refers to determining the level of afirst form of a vitamin or polypeptide by measuring or detecting thelevel of the first form of a vitamin or polypeptide as opposed tocalculating the level of the first form of a vitamin or polypeptideusing the level of a second or further form of a vitamin or polypeptidewhich was directly determined.

Direct measurement of the level of free and/or bioavailable vitamin Dcan be accomplished by a number of methods. Non-limiting examples ofmethods for direct measurement of the level of free vitamin D includethe following. 1) Vitamin D complexed with VDBP can be depleted from asample using immunodepletion or affinity binding chromatography todeplete VDBP from the sample. The remaining vitamin D (the bioavailablefraction) can then be measured according to any of the methods describedelsewhere herein. 2) Vitamin D complexed with VDBP can be depleted froma sample using differential precipitation of VDBP with antibodies(immunoprecipitation), actin (affinity precipitation) with or withoutprecipitating buffers (e.g. polyethylene glycol, ammonium sulfate)followed by centrifugal separation from free and/or bioavailable vitaminD and measurement of remaining vitamin D fraction according to any ofthe methods described elsewhere herein. 3) Immunoassays can also be used(see for example, Ekins et al. J Endocrinol Invest. 9 Suppl 4:3-15.1986; which is incorporated by reference herein in its entirety).Immunoassays capitalize on the idea that free analyte may be measuredusing lower affinity antibodies which do not “strip” the vitamin fromits high affinity binding protein. Free and/or bioavailable vitamin Dcan be measured directly using several competitive immunoassayapproaches described below and in (Christofides, Nic D. The ImmunoassayHandbook, 3^(rd) Ed. Editor David Wild, Elsevier Ltd, 2005; which isincorporated by reference herein in its entirety) These methods are allbased upon two principals: (1) Antibodies will differentiate betweenfree and protein-bound analytes if the reaction conditions (pH,temperature, buffers) do not interfere with binding between the vitaminand its binding protein, and (2) The total antibody binding capacity(affinity constant times antibody concentration) does not significantlydeplete the total vitamin D concentration and thus does not “strip” theprotein-bound vitamin D. This may be achieved using an antibody withrelatively low affinity (˜10¹⁰ L/M) for the vitamin D ligand and/orlimited assay reaction times which allow for binding of free andalbumin-bound vitamin D but are too short to allow dissociation ofVDBP-bound vitamin D.

Specific immunoassay designs that can be used include the following: A)Two-step measurement of free and/or bioavailable vitamin D with antibodycapture—(1) capture of free and/or bioavailable vitamin D withimmobilized vitamin D-binding antibody, (2) wash away unbound vitamin Dand VDBP, (3) detection of bound vitamin D by competitive binding withlabeled vitamin D (or labeled vitamin D analog that also binds theantibody). B) One-step measurement of free and/or bioavailable vitamin Dwith immobilized vitamin D-binding antibody and competitive detectionusing labeled vitamin D (or labeled vitamin D analog). C) One-stepmeasurement of free and/or bioavailable vitamin D with immobilizedvitamin D or vitamin D analog and labeled vitamin D-binding antibody.Free and/or bioavailable vitamin D from a sample competes with theimmobilized vitamin D for binding to the labeled antibody. D) One-stepmeasurement of free and/or bioavailable vitamin D with fluorescencepolarization immunoassay (Mendel CM. Clin Chem. 38(9):1916-7. 1992;which is incorporated by reference herein in its entirety). Free and/orbioavailable vitamin D and fluorescently labeled vitamin D analogcompete for binding to antibody and polarized fluorescence indicatesrelative amount of free and/or bioavailable vitamin D competing forbinding sites.

In some embodiments, determining of the level of free and/orbioavailable vitamin D can comprise the use of a method selected fromthe group consisting of: immunoassay; two-step immunoassay with antibodycapture; one-step immunoassay with immobilized antibody and competitivedetection; one-step immunoassay with immobilized competitior and labeledantibody; fluorescence polarization immunoassay; differentialprecipitation (immunoprecipitation, affinity precipitation);immunodepletion; and affinity binding chromatography; and a methodselected from the group consisting of: radioimmunoassay;chemiluminescent immunosorbent assay; electrochemiluminescentimmunosorbent assay; fluorescent immunosorbent assay; dye linkedimmunosorbent assay; liquid chromatography tandem mass spectroscopy andhigh-pressure liquid chromatography.

In some embodiments, determining the level of free and/or bioavailablevitamin D is performed directly using a differential precipitationassay. As noted above, VDBP binds vitamin D tightly, with aK_(D)=0.7×10⁹ M⁻¹ (for human VDBP). Albumin binds vitamin D less tightlythan VDBP, with a K_(D)=6×10⁵ M⁻¹ (for human albumin). This differencein affinity is exploited in an assay in which the high-affinity VDBP isused to pull free and albumin-bound D out of a subject sample, andlabeled D is used to compete and quantify the amount of free andalbumin-bound D in the sample.

An exemplary differential precipication assay is shown in FIG. 13. Insome embodiments, purified VDBP is immobilized by coating it onto asurface, e.g., a bead or well of a microtiter plate. The subject sampleis applied, preferably after dilution into a suitable buffer, e.g., PBS.After a time sufficient to allow the D in the sample to equilibrate ontothe immobilized VDBP, the liquid in the sample can optionally beremoved. A labeled, purified D is then applied (whether or not theliquid sample was removed), and again allowed to equilibrate. Afterequilibration, unbound labeled D is removed by washing the immobilizedVDBP with appropriate buffer. The amount of affinity precipitatedlabeled D is then determined using a method appropriate for the label;in a preferred embodiment, the labeled D is radiolabeled and the amountof radiolabled D is determined using scintillation counting. In otherembodiments, the D is fluorescently labeled (see, e.g., Shimizu et al.,Anal Biochem. 1991 April; 194(1):77-81); alternatively the D can belabeled with a radiologically detectable tag; fluorescent tag;luminescent tag; or a colorimetric tag. The amount of immobilizedlabeled D is then used to determine the amount of free and albumin-boundvitamin D present in the subject sample; the fraction of labeled vitaminD competitor reagent that is immobilized indicates the level of vitaminD in the blood sample which is bioavailable

The labeled D can be, e.g., 25-hydroxyvitamin D, 1,25-dihydroxyvitaminD, or 24,25-dihydroxivitamin D. In some embodiments the vitamin D ischemically labeled 25-hydrovitamin D3 or vitamin D analogs with VDBPbinding affinity. These chemical modifications to incorporate labelingmoieties can include succinylation of the 3-hydroxyl group to yield25-hydroxyvitamin D-3-hemisuccinate (Bouillon et al., Clin Chem. 1984November; 30(11):1731-6). This succinylation modification provides alinker group ending with a carboxyl moiety which may be furtherderivatized with carboxyl-reactive compounds that are chromatophoric,fluorescent, luminescent, chemiluminescent, or linked to enzymes. Thelabeling reaction of the carboxyl group on 25-hydroxyvitaminD-3-hemisuccinate can be modified directly, e.g., with labeling moietiescontaining diazoalkanes or alkyl halides: they can also be labeled byhydrazines, hydroxylamines, or primary amines in the presence ofcarbodiimide.

In some embodiments, purified vitamin D, e.g., 25-hydroxyvitamin D,1,25-dihydroxyvitamin D, or 24,25-dihydroxivitamin D, is immobilized,rather than the VDBP, and the VDBP is labeled.

In some embodiments, the methods include simply using a one-stephomogeneous binding assay with fluorescence polarization detection ofbound ligand. A schematic of an exemplary assay for vitamin D usingfluorescence polarization is provided in FIG. 14. Fluorescencepolarization is based on the principal wherein fluorophores are excitedby polarized light of the appropriate wavelength, and if the rotationaldiffusion (via Brownian motion) of the fluorophore is slower than thehalf-life of its fluorescent emission, the light emitted retains thesame angle of polarization as the excitation photon. In theseembodiments, 25-hydroxyvitamin D is attached to a fluorescent label andadded to patient serum or plasma diluted in appropriate buffer. Thelabeled ligand binds tightly to VDBP in the patient sample, slowing itsrate of rotational diffusion. The free and albumin-associated labeled D,in contrast, is less restricted and thus has faster rates of rotationaldiffusion. The fluorescent labels on free and bound ligands are excitedwith polarized light of the appropriate wavelength, resulting influorescent emission at a second wavelength. When the labeled D is boundto VDBP, its decreased rate of rotational diffusion causes the emittedfluorescence to retain the same angle of polarity as the excitationphotons. The emitted light is then filtered with an appropriatelypolarization filter, and the polarized fluorescent emission are measuredwith a photometer. In this way, the amounts of polarized fluorescentemissions are proportional to the amounts of labeled D that have boundto VDBP in patients' samples. The amount of unpolarized fluorescence, incontrast, is proportional to the concentrations of non-VDBP bound25-hydroxyvitamin D. The ratio of unpolarized and polarized fluorescenceis thusly proportional to the ratio of bioavailable versus VDBP-bound25-hydroxyvitamin D. This methodology has been used to measureconcentrations of serum free thyroid hormone (Mendel, Clin Chem. 1992September; 38(9):1916-7); binding of fluorescently labeled estradiol toestrogen receptor (Parker et al., J Biomol Screen. 2000 April;5(2):77-88); and serum free drug concentrations (Mathias and Jung, AnalBioanal Chem. 2007 July; 388(5-6):1147-56). In some embodiments, therotational diffusion of VDBP-bound labeled D may be further reduced byadding anti-VDBP antibodies to this homogeneous assay in order tofurther reduce the rotational diffusion rate of VDBP.

In some embodiments, the methods include determining the total amount ofvitamin D present in the sample, e.g., using a method described herein.

In some embodiments, when the level of bioavailable or free vitamin Ddetermined to be in the blood sample of a subject is lower than athreshold level, e.g., the 25th percentile value or 25% of the meanvalue, of bioavailable or free vitamin D in a population of healthysubjects, the subject is likely to have a vitamin D insufficiency. Insome embodiments, when the level of bioavailable or free vitamin Ddetermined to be in the blood sample of a subject is lower than athreshold level, e.g., the 25th percentile value or 25% of the meanvalue, of bioavailable or free vitamin D in a population of healthysubjects, the subject is indicated to have a vitamin D insufficiency. Insome embodiments, when the level of bioavailable or free vitamin Ddetermined to be in the blood sample of a subject is lower than athreshold level, e.g., the 25th percentile value or 25% of the meanvalue, of bioavailable or free vitamin D in a population of healthysubjects, the subject has an increased likelihood of having ordeveloping a vitamin D-associated disease. In some embodiments, when thelevel of bioavailable or free vitamin D determined to be in the bloodsample of a subject is lower than a threshold level, e.g., the 25thpercentile value or 25% of the mean value, of bioavailable or freevitamin D in a population of healthy subjects, the subject is in need ofa treatment for vitamin D insufficiency. In some embodiments, when thelevel of bioavailable or free vitamin D determined to be in the bloodsample of a subject is lower than a threshold level, e.g., the 25thpercentile value or 25% of the mean value, of bioavailable or freevitamin D in a population of healthy subjects, the subject has a greaterlikelihood of being in need of a treatment for vitamin D insufficiency.

In some embodiments, when the level of bioavailable or free vitamin Ddetermined to be in the blood sample of a subject is above a thresholdlevel, e.g., the 25th percentile value, or than 25% of the mean value,of bioavailable or free vitamin D in a population of healthy subjects,the subject is not treated for vitamin D insufficiency.

Methods of Determining a VDBP Genotype in a Subject

In some aspects, the methods described herein include determining a VDBPgenotype in a subject. As described herein, the VDBP genotype affectsnormal circulating concentrations of plasma VDBP as well as the bindingaffinity of the VDBP for vitamin D, and therefor affects the clinicalrelevance of determination of vitamin D levels.

The methods described herein include determining the VDBP genotype of asubject. In some embodiments, a VDBP genotype is determined by detectingthe identity of both alleles of two common single nucleotidepolymorphisms in the VDBP gene (rs4588 and rs7041) in an subject. Thusthe methods can include obtaining and analyzing a sample from a subject.The SNPs, plus flanking sequences, are shown in the following table A:

TABLE A SEQ ID SNP SEQUENCE NO: rs4588 AGCAAAATTGCCTGATGCCACACCCA[A/C] 7GGAACTGGCAAAGCTGGTTAACAAG rs7041 GAGCGACTAAAAGCAAAATTGCCTGA[G/T] 8GCCACACCCACGGAACTGGCAAAGC

The following genotypes have been identified:

Gc1S variant of the Vitamin D binding protein, encoded by the GC genecontaining the rs7041 single nucleotide polymorphisms, with a T>Gsubstitution resulting in the substitution of Aspartic acid withglutamic acid at residue 416 of the VDBP polypeptide. The rs4588 singlenucleotide polymorphism for Gc1S (C) is the ancestral allele encodingthreonine at position 420.

Gc1F variant of the Vitamin D binding protein, encoded by the GC genecontaining the ancestral alleles for both rs7041 and rs4588 singlenucleotide polymorphisms; these alleles encode for aspartic acid andthreonine at positions 416 and 420 of the VDBP polypeptide.

Gc2 variant of the Vitamin D binding protein, encoded by the GC genecontaining the rs4588 SNP, with a C>A substitution resulting in thesubstitution of threonine acid with lysine at residue 420 of the VDBPpolypeptide. The rs7041 single nucleotide polymorphism for Gc2 is theancestral allele (T) encoding aspartic acid at position 416.

Gc1S variant of the Vitamin D binding protein, encoded by the GC genecontaining the rs7041 SNP, with a T>G substitution resulting in thesubstitution of Aspartic acid with glutamic acid at residue 416 of theVDBP polypeptide.

Once the VDBP genotype for an individual subject is known, for patientswho are homozygous for Gc1S/Gc1S, Gc1F/Gc1F, or Gc2/Gc2, thegenotype-adjusted free and bioavailable fractions of 25-hydroxyvitamin Dcan be calculated using the known binding affinities for the threevariants (Lauridsen et al., Clin Chem 2001; 47:753-6):

For subjects homozygous for Gc1F variant, KDBP=1.12×108M-1

For subjects homozygous for Gc1S variant, KDBP=0.60×108M-1

For subjects homozygous for Gc2 variant, KDBP=0.36×108M-1

Samples that are suitable for use in the methods described hereincontain genetic material, e.g., genomic DNA (gDNA). Genomic DNA istypically extracted from biological samples such as blood or mucosalscrapings of the lining of the mouth, but can be extracted from otherbiological samples including urine or expectorant. The sample itselfwill typically consist of nucleated cells (e.g., blood or buccal cells)or tissue removed from the subject. The subject can be an adult, child,fetus, or embryo. In some embodiments, the sample is obtainedprenatally, either from a fetus or embryo or from the mother (e.g., fromfetal or embryonic cells in the maternal circulation). Methods andreagents are known in the art for obtaining, processing, and analyzingsamples. In some embodiments, the sample is obtained with the assistanceof a health care provider, e.g., to draw blood. In some embodiments, thesample is obtained without the assistance of a health care provider,e.g., where the sample is obtained non-invasively, such as a samplecomprising buccal cells that is obtained using a buccal swab or brush,or a mouthwash sample. In some embodiments, the same sample that is usedfor the determination of free and/or bioavailable vitamin D is also usedto detect the VDBP genotype in the subject.

In some cases, a biological sample may be processed for DNA isolation.For example, DNA in a cell or tissue sample can be separated from othercomponents of the sample. Cells can be harvested from a biologicalsample using standard techniques known in the art. For example, cellscan be harvested by centrifuging a cell sample and resuspending thepelleted cells. The cells can be resuspended in a buffered solution suchas phosphate-buffered saline (PBS). After centrifuging the cellsuspension to obtain a cell pellet, the cells can be lysed to extractDNA, e.g., gDNA. See, e.g., Ausubel et al., 2003, supra. The sample canbe concentrated and/or purified to isolate DNA. All samples obtainedfrom a subject, including those subjected to any sort of furtherprocessing, are considered to be obtained from the subject. Routinemethods can be used to extract genomic DNA from a biological sample,including, for example, phenol extraction. Alternatively, genomic DNAcan be extracted with kits such as the QIAamp® Tissue Kit (Qiagen,Chatsworth, Calif.) and the Wizard® Genomic DNA purification kit(Promega). Non-limiting examples of sources of samples include urine,blood, and tissue.

The absence or presence of an allele as described herein can bedetermined using methods known in the art. For example, gelelectrophoresis, capillary electrophoresis, size exclusionchromatography, sequencing, and/or arrays can be used to detect thepresence or absence of the allele or genotype. Amplification of nucleicacids, where desirable, can be accomplished using methods known in theart, e.g., PCR. In one example, a sample (e.g., a sample comprisinggenomic DNA), is obtained from a subject. The DNA in the sample is thenexamined to identify or detect the presence of an allele or genotype asdescribed herein. The allele or genotype can be identified or determinedby any method described herein, e.g., by sequencing or by hybridizationof the gene in the genomic DNA, RNA, or cDNA to a nucleic acid probe,e.g., a DNA probe (which includes cDNA and oligonucleotide probes) or anRNA probe. The nucleic acid probe can be designed to specifically orpreferentially hybridize with a particular polymorphic variant.

Other methods of nucleic acid analysis can include direct manualsequencing (Church and Gilbert, Proc. Natl. Acad. Sci. USA 81:1991-1995(1988); Sanger et al., Proc. Natl. Acad. Sci. USA 74:5463-5467 (1977);Beavis et al., U.S. Pat. No. 5,288,644); automated fluorescentsequencing; single-stranded conformation polymorphism assays (SSCP)(Schafer et al., Nat. Biotechnol. 15:33-39 (1995)); clamped denaturinggel electrophoresis (CDGE); two-dimensional gel electrophoresis (2DGE orTDGE); conformational sensitive gel electrophoresis (CSGE); denaturinggradient gel electrophoresis (DGGE) (Sheffield et al., Proc. Natl. Acad.Sci. USA 86:232-236 (1989)); denaturing high performance liquidchromatography (DHPLC, Underhill et al., Genome Res. 7:996-1005 (1997));infrared matrix-assisted laser desorption/ionization (IR-MALDI) massspectrometry (WO 99/57318); mobility shift analysis (Orita et al., Proc.Natl. Acad. Sci. USA 86:2766-2770 (1989)); restriction enzyme analysis(Flavell et al., Cell 15:25 (1978); Geever et al., Proc. Natl. Acad.Sci. USA 78:5081 (1981)); quantitative real-time PCR (Raca et al., GenetTest 8(4):387-94 (2004)); heteroduplex analysis; chemical mismatchcleavage (CMC) (Cotton et al., Proc. Natl. Acad. Sci. USA 85:4397-4401(1985)); RNase protection assays (Myers et al., Science 230:1242(1985)); use of polypeptides that recognize nucleotide mismatches, e.g.,E. coli mutS protein; allele-specific PCR, and combinations of suchmethods. See, e.g., Gerber et al., U.S. Patent Publication No.2004/0014095 which is incorporated herein by reference in its entirety.

Sequence analysis can also be used to detect specific polymorphicvariants. For example, polymorphic variants can be detected bysequencing exons, introns, 5′ untranslated sequences, or 3′ untranslatedsequences. A sample comprising DNA or RNA is obtained from the subject.PCR or other appropriate methods can be used to amplify a portionencompassing the polymorphic site, if desired. The sequence is thenascertained, using any standard method, and the presence of apolymorphic variant is determined. Real-time pyrophosphate DNAsequencing is yet another approach to detection of polymorphisms andpolymorphic variants (Alderborn et al., Genome Research 10(8):1249-1258(2000)). Additional methods include, for example, PCR amplification incombination with denaturing high performance liquid chromatography(dHPLC) (Underhill et al., Genome Research 7(10):996-1005 (1997)).

In order to detect polymorphisms and/or polymorphic variants, it willfrequently be desirable to amplify a portion of genomic DNA (gDNA)encompassing the polymorphic site. Such regions can be amplified andisolated by PCR using oligonucleotide primers designed based on genomicand/or cDNA sequences that flank the site. PCR refers to procedures inwhich target nucleic acid (e.g., genomic DNA) is amplified in a mannersimilar to that described in U.S. Pat. No. 4,683,195, and subsequentmodifications of the procedure described therein. Generally, sequenceinformation from the ends of the region of interest or beyond are usedto design oligonucleotide primers that are identical or similar insequence to opposite strands of a potential template to be amplified.See e.g., PCR Primer: A Laboratory Manual, Dieffenbach and Dveksler,(Eds.); McPherson et al., PCR Basics: From Background to Bench (SpringerVerlag, 2000); Mattila et al., Nucleic Acids Res., 19:4967 (1991);Eckert et al., PCR Methods and Applications, 1:17 (1991); PCR (eds.McPherson et al., IRL Press, Oxford); and U.S. Pat. No. 4,683,202. Otheramplification methods that may be employed include the ligase chainreaction (LCR) (Wu and Wallace, Genomics 4:560 (1989), Landegren et al.,Science 241:1077 (1988), transcription amplification (Kwoh et al., Proc.Natl. Acad. Sci. USA 86:1173 (1989)), self-sustained sequencereplication (Guatelli et al., Proc. Nat. Acad. Sci. USA 87:1874 (1990)),and nucleic acid based sequence amplification (NASBA). Guidelines forselecting primers for PCR amplification are well known in the art. See,e.g., McPherson et al., PCR Basics: From Background to Bench,Springer-Verlag, 2000. A variety of computer programs for designingprimers are available, e.g., ‘Oligo’ (National Biosciences, Inc,Plymouth Minn.), MacVector (Kodak/IBI), and the GCG suite of sequenceanalysis programs (Genetics Computer Group, Madison, Wis. 53711).

In some cases, PCR conditions and primers can be developed that amplifya product only when the variant allele is present or only when the wildtype allele is present (MSPCR or allele-specific PCR). For example,patient DNA and a control can be amplified separately using either awild type primer or a primer specific for the variant allele. Each setof reactions is then examined for the presence of amplification productsusing standard methods to visualize the DNA. For example, the reactionscan be electrophoresed through an agarose gel and the DNA visualized bystaining with ethidium bromide or other DNA intercalating dye. In DNAsamples from heterozygous patients, reaction products would be detectedin each reaction.

Real-time quantitative PCR can also be used to determine copy number.Quantitative PCR permits both detection and quantification of specificDNA sequence in a sample as an absolute number of copies or as arelative amount when normalized to DNA input or other normalizing genes.A key feature of quantitative PCR is that the amplified DNA product isquantified in real-time as it accumulates in the reaction after eachamplification cycle. Methods of quantification can include the use offluorescent dyes that intercalate with double-stranded DNA, and modifiedDNA oligonucleotide probes that fluoresce when hybridized with acomplementary DNA. Methods of quantification can include determining theintensity of fluorescence for fluorescently tagged molecular probesattached to a solid surface such as a microarray.

In some embodiments, a peptide nucleic acid (PNA) probe can be usedinstead of a nucleic acid probe in the hybridization methods describedabove. PNA is a DNA mimetic with a peptide-like, inorganic backbone,e.g., N-(2-aminoethyl)glycine units, with an organic base (A, G, C, T orU) attached to the glycine nitrogen via a methylene carbonyl linker(see, e.g., Nielsen et al., Bioconjugate Chemistry, The AmericanChemical Society, 5:1 (1994)). The PNA probe can be designed tospecifically hybridize to a nucleic acid comprising a polymorphicvariant conferring susceptibility to or indicative of the presence of agiven VBDP genotype.

In some cases, allele-specific oligonucleotides can also be used todetect the presence of a polymorphic variant. For example, polymorphicvariants can be detected by performing allele-specific hybridization orallele-specific restriction digests. Allele specific hybridization is anexample of a method that can be used to detect sequence variants,including complete genotypes of a subject (e.g., a mammal such as ahuman). See Stoneking et al., Am. J. Hum. Genet. 48:370-382 (1991); andPrince et al., Genome Res. 11:152-162 (2001). An “allele-specificoligonucleotide” (also referred to herein as an “allele-specificoligonucleotide probe”) is an oligonucleotide that is specific forparticular a polymorphism can be prepared using standard methods (seeAusubel et al., Current Protocols in Molecular Biology, supra).Allele-specific oligonucleotide probes typically can be approximately10-50 base pairs, preferably approximately 15-30 base pairs, thatspecifically hybridizes to a nucleic acid region that contains apolymorphism. Hybridization conditions are selected such that a nucleicacid probe can specifically bind to the sequence of interest, e.g., thevariant nucleic acid sequence. Such hybridizations typically areperformed under high stringency as some sequence variants include only asingle nucleotide difference. In some cases, dot-blot hybridization ofamplified oligonucleotides with allele-specific oligonucleotide (ASO)probes can be performed. See, for example, Saiki et al., Nature (London)324:163-166 (1986).

In some embodiments, allele-specific restriction digest analysis can beused to detect the existence of a polymorphic variant of a polymorphism,if alternate polymorphic variants of the polymorphism result in thecreation or elimination of a restriction site. Allele-specificrestriction digests can be performed in the following manner. A samplecontaining genomic DNA is obtained from the individual and genomic DNAis isolated for analysis. For nucleotide sequence variants thatintroduce a restriction site, restriction digest with the particularrestriction enzyme can differentiate the alleles. In some cases,polymerase chain reaction (PCR) can be used to amplify a regioncomprising the polymorphic site, and restriction fragment lengthpolymorphism analysis is conducted (see Ausubel et al., CurrentProtocols in Molecular Biology, supra). The digestion pattern of therelevant DNA fragment indicates the presence or absence of a particularpolymorphic variant of the polymorphism and is therefore indicative ofthe presence of a specific VDBP genotype. For sequence variants that donot alter a common restriction site, mutagenic primers can be designedthat introduce a restriction site when the variant allele is present orwhen the wild type allele is present. For example, a portion of anucleic acid can be amplified using the mutagenic primer and a wild typeprimer, followed by digest with the appropriate restrictionendonuclease.

In some embodiments, fluorescence polarization template-directeddye-terminator incorporation (FP-TDI) is used to determine which ofmultiple polymorphic variants of a polymorphism is present in a subject(Chen et al., Genome Research 9(5):492-498 (1999)). Rather thaninvolving use of allele-specific probes or primers, this method employsprimers that terminate adjacent to a polymorphic site, so that extensionof the primer by a single nucleotide results in incorporation of anucleotide complementary to the polymorphic variant at the polymorphicsite.

In some cases. DNA containing an amplified portion may be dot-blotted,using standard methods (see Ausubel et al., Current Protocols inMolecular Biology, supra), and the blot contacted with theoligonucleotide probe. The presence of specific hybridization of theprobe to the DNA is then detected. Specific hybridization of anallele-specific oligonucleotide probe (specific for a polymorphicvariant shown in Table A) to DNA from the subject is indicative of thepresence of a given VDBP genotype.

The methods typically include determining the genotype of a subject withrespect to both copies of the polymorphic site present in the genome.For example, the complete genotype may be characterized as −/−, as −/+,or as +/+, where a minus sign indicates the presence of the reference orwild type sequence at the polymorphic site, and the plus sign indicatesthe presence of a polymorphic variant other than the reference sequence.If multiple polymorphic variants exist at a site, this can beappropriately indicated by specifying which ones are present in thesubject. Any of the detection means described herein can be used todetermine the genotype of a subject with respect to one or both copiesof the polymorphism present in the subject's genome.

Additional methods of nucleic acid analysis to detect polymorphismsand/or polymorphic variants can include. e.g., microarray analysis. Insome embodiments, it is desirable to employ methods that can detect thepresence of multiple polymorphisms (e.g., polymorphic variants at aplurality of polymorphic sites) in parallel or substantiallysimultaneously. Oligonucleotide arrays represent one suitable means fordoing so. Other methods, including methods in which reactions (e.g.,amplification, hybridization) are performed in individual vessels, e.g.,within individual wells of a multi-well plate or other vessel may alsobe performed so as to detect the presence of multiple polymorphicvariants (e.g., polymorphic variants at a plurality of polymorphicsites) in parallel or substantially simultaneously according to certainembodiments.

Nucleic acid probes can be used to detect and/or quantify the presenceof a particular target nucleic acid sequence within a sample of nucleicacid sequences, e.g., as hybridization probes, or to amplify aparticular target sequence within a sample, e.g., as a primer. Probeshave a complimentary nucleic acid sequence that selectively hybridizesto the target nucleic acid sequence. In order for a probe to hybridizeto a target sequence, the hybridization probe must have sufficientidentity with the target sequence, i.e., at least 70% (e.g., 80%, 90%,95%, 98% or more) identity to the target sequence. The probe sequencemust also be sufficiently long so that the probe exhibits selectivityfor the target sequence over non-target sequences. For example, theprobe will be at least 20 (e.g., 25, 30, 35, 50, 100, 200, 300, 400,500, 600, 700, 800, 900 or more) nucleotides in length. In someembodiments, the probes are not more than 30, 50, 100, 200, 300, 500,750, or 1000 nucleotides in length. Probes are typically about 20 toabout 1×106 nucleotides in length. Probes include primers, whichgenerally refers to a single-stranded oligonucleotide probe that can actas a point of initiation of template-directed DNA synthesis usingmethods such as PCR (polymerase chain reaction), LCR (ligase chainreaction), etc., for amplification of a target sequence.

The probe can be a test probe such as a probe that can be used to detectpolymorphisms in a region described herein (e.g., polymorphisms asdescribed herein). For example, the probe can hybridize to an alleledescribed herein, e.g., in Table A.

Control probes can also be used. For example, a probe that binds a lessvariable sequence, e.g., repetitive DNA associated with a centromere ofa chromosome, can be used as a control. Probes that hybridize withvarious centromeric DNA and locus-specific DNA are availablecommercially, for example, from Vysis, Inc. (Downers Grove, Ill.),Molecular Probes, Inc. (Eugene, Oreg.), or from Cytocell (Oxfordshire,UK). Probe sets are available commercially such from Applied Biosystems,e.g., the Assays-on-Demand SNP kits Alternatively, probes can besynthesized, e.g., chemically or in vitro, or made from chromosomal orgenomic DNA through standard techniques. For example, sources of DNAthat can be used include genomic DNA, cloned DNA sequences, somatic cellhybrids that contain one, or a part of one, human chromosome along withthe normal chromosome complement of the host, and chromosomes purifiedby flow cytometry or microdissection. The region of interest can beisolated through cloning, or by site-specific amplification via thepolymerase chain reaction (PCR). See, for example, Nath and Johnson,Biotechnic. Histochem. 73(1):6-22 (1998); Wheeless et al., Cytometry17:319-326 (1994); and U.S. Pat. No. 5,491,224.

In some embodiments, the probes are labeled, e.g., by direct labeling,with a fluorophore, an organic molecule that fluoresces after absorbinglight of lower wavelength/higher energy. A directly labeled fluorophoreallows the probe to be visualized without a secondary detectionmolecule. After covalently attaching a fluorophore to a nucleotide, thenucleotide can be directly incorporated into the probe with standardtechniques such as nick translation, random priming, and PCR labeling.Alternatively, deoxycytidine nucleotides within the probe can betransaminated with a linker. The fluorophore then is covalently attachedto the transaminated deoxycytidine nucleotides. See, e.g., U.S. Pat. No.5,491,224.

Fluorophores of different colors can be chosen such that each probe in aset can be distinctly visualized. For example, a combination of thefollowing fluorophores can be used: 7-amino-4-methylcoumarin-3-aceticacid (AMCA), TEXAS RED™ (Molecular Probes, Inc., Eugene, Oreg.),5-(and-6)-carboxy-X-rhodanine, lissamine rhodamine B,5-(and-6)-carboxyfluorescein, fluorescein-5-isothiocyanate (FITC),7-diethylaminocoumarin-3-carboxylic acid,tetramethylrhodamine-5-(and-6)-isothiocyanate,5-(and-6)-carboxytetramethylrhodamine, 7-hydroxycoumarin-3-carboxylicacid, 6-[fluorescein 5-(and-6)-carboxamido]hexanoic acid,N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a diaza-3-indacenepropionicacid, eosin-5-isothiocyanate, erythrosin-5-isothiocyanate, and CASCADE™blue acetylazide (Molecular Probes, Inc., Eugene, Oreg.). Fluorescentlylabeled probes can be viewed with a fluorescence microscope and anappropriate filter for each fluorophore, or by using dual or tripleband-pass filter sets to observe multiple fluorophores. See, forexample, U.S. Pat. No. 5,776,688. Alternatively, techniques such as flowcytometry can be used to examine the hybridization pattern of theprobes. Fluorescence-based arrays are also known in the art.

In other embodiments, the probes can be indirectly labeled with, e.g.,biotin or digoxygenin, or labeled with radioactive isotopes such as 32Pand 3H. For example, a probe indirectly labeled with biotin can bedetected by avidin conjugated to a detectable marker. For example,avidin can be conjugated to an enzymatic marker such as alkalinephosphatase or horseradish peroxidase. Enzymatic markers can be detectedin standard colorimetric reactions using a substrate and/or a catalystfor the enzyme. Catalysts for alkaline phosphatase include5-bromo-4-chloro-3-indolylphosphate and nitro blue tetrazolium.Diaminobenzoate can be used as a catalyst for horseradish peroxidase.

In another aspect, this document features arrays that include asubstrate having a plurality of addressable areas, and methods of usingthem. At least one area of the plurality includes a nucleic acid probethat binds specifically to a sequence comprising a polymorphism listedin Tables 1-2 or Table A, and can be used to detect the absence orpresence of said polymorphism, e.g., one or more SNPs, microsatellites,minisatellites, or indels, as described herein, to determine or identifyan allele or genotype. For example, the array can include one or morenucleic acid probes that can be used to detect a polymorphism listed inTable A. In some embodiments, the probes are nucleic acid captureprobes.

Generally, microarray hybridization is performed by hybridizing anucleic acid of interest (e.g., a nucleic acid encompassing apolymorphic site) with the array and detecting hybridization usingnucleic acid probes. In some cases, the nucleic acid of interest isamplified prior to hybridization. Hybridization and detecting aregenerally carried out according to standard methods. See, e.g.,Published PCT Application Nos. WO 92/10092 and WO 95/11995, and U.S.Pat. No. 5,424,186. For example, the array can be scanned to determinethe position on the array to which the nucleic acid hybridizes. Thehybridization data obtained from the scan is typically in the form offluorescence intensities as a function of location on the array.

Arrays can be formed on substrates fabricated with materials such aspaper, glass, plastic (e.g., polypropylene, nylon, or polystyrene),polyacrylamide, nitrocellulose, silicon, optical fiber, or any othersuitable solid or semisolid support, and can be configured in a planar(e.g., glass plates, silicon chips) or three dimensional (e.g., pins,fibers, beads, particles, microtiter wells, capillaries) configuration.Methods for generating arrays are known in the art and include, e.g.,photolithographic methods (see, e.g., U.S. Pat. Nos. 5,143,854;5,510,270; and 5,527,681), mechanical methods (e.g., directed-flowmethods as described in U.S. Pat. No. 5,384,261), pin-based methods(e.g., as described in U.S. Pat. No. 5,288,514), and bead-basedtechniques (e.g., as described in PCT US/93/04145). The array typicallyincludes oligonucleotide hybridization probes capable of specificallyhybridizing to different polymorphic variants. Oligonucleotide probesthat exhibit differential or selective binding to polymorphic sites mayreadily be designed by one of ordinary skill in the art. For example, anoligonucleotide that is perfectly complementary to a sequence thatencompasses a polymorphic site (i.e., a sequence that includes thepolymorphic site, within it or at one end) will generally hybridizepreferentially to a nucleic acid comprising that sequence, as opposed toa nucleic acid comprising an alternate polymorphic variant.

Oligonucleotide probes forming an array may be attached to a substrateby any number of techniques, including, without limitation, (i) in situsynthesis (e.g., high-density oligonucleotide arrays) usingphotolithographic techniques; (ii) spotting/printing at medium to lowdensity on glass, nylon or nitrocellulose; (iii) by masking, and (iv) bydot-blotting on a nylon or nitrocellulose hybridization membrane.Oligonucleotides can be immobilized via a linker, including by covalent,ionic, or physical linkage. Linkers for immobilizing nucleic acids andpolypeptides, including reversible or cleavable linkers, are known inthe art. See, for example, U.S. Pat. No. 5,451,683 and WO98/20019.Alternatively, oligonucleotides can be non-covalently immobilized on asubstrate by hybridization to anchors, by means of magnetic beads, or ina fluid phase such as in microtiter wells or capillaries. Immobilizedoligonucleotide probes are typically about 20 nucleotides in length, butcan vary from about 10 nucleotides to about 1000 nucleotides in length.

Arrays can include multiple detection blocks (i.e., multiple groups ofprobes designed for detection of particular polymorphisms). Such arrayscan be used to analyze multiple different polymorphisms. Detectionblocks may be grouped within a single array or in multiple, separatearrays so that varying conditions (e.g., conditions optimized forparticular polymorphisms) may be used during the hybridization. Forexample, it may be desirable to provide for the detection of thosepolymorphisms that fall within G-C rich stretches of a genomic sequence,separately from those falling in A-T rich segments. General descriptionsof using oligonucleotide arrays for detection of polymorphisms can befound, for example, in U.S. Pat. Nos. 5,858,659 and 5,837,832. Inaddition to oligonucleotide arrays, cDNA arrays may be used similarly incertain embodiments.

The methods described herein can include providing an array as describedherein; contacting the array with a sample (e.g., a portion of genomicDNA that includes at least a portion of a human chromosome), anddetecting binding of a nucleic acid from the sample to the array.Optionally, the method includes amplifying nucleic acid from the sample,e.g., genomic DNA that includes a portion of a human chromosomedescribed herein prior to or during contact with the array.

In some aspects, the methods described herein can include using an arraythat can ascertain differential expression patterns or copy numbers ofone or more genes in samples from normal and affected individuals (see,e.g., Redon et al., Nature 444(7118):444-54 (2006)). For example, arraysof probes to a marker described herein can be used to measurepolymorphisms between DNA from a subject having a specific VBDPgenotype, and control DNA, e.g., DNA obtained from an individual thatdoes not that VBDP genotype. Since the clones on the array containsequence tags, their positions on the array are accurately knownrelative to the genomic sequence. Methods for array production,hybridization, and analysis are described, e.g., in Snijders et al.,Nat. Genetics 29:263-264 (2001); Klein et al., Proc. Natl Acad. Sci. USA96:4494-4499 (1999); Albertson et al., Breast Cancer Research andTreatment 78:289-298 (2003); and Snijders et al., “BAC microarray basedcomparative genomic hybridization,” in: Zhao et al. (eds), BacterialArtificial Chromosomes: Methods and Protocols, Methods in MolecularBiology, Humana Press, 2002.

In another aspect, this document provides methods of determining theabsence or presence of a VBDP genotype as described herein, using anarray described above. The methods can include providing a twodimensional array having a plurality of addresses, each address of theplurality being positionally distinguishable from each other address ofthe plurality having a unique nucleic acid capture probe, contacting thearray with a sample from a test subject, and analyzing the binding ofthe sample to determine the VBDP genotype in the subject. In the case ofa nucleic acid hybridization, binding with a capture probe at an addressof the plurality, can be detected by any method known in the art, e.g.,by detection of a signal generated from a label attached to the nucleicacid.

Alternatively or in addition, a subject's VDBP genotype can bedetermined by analysis of the protein product of the VDBP gene in thesubject. Thus, for example, analysis of patients' VDBP variant genotypemay be achieved by analyzing for the presence of the Gc1F, Gc1S, or Gc2protein variants using chromatography, mass spectrometry, or antibodiesdirected against the specific variants.

A subject's VDBP genotype can be used in determining the reference valueto which a level of total vitamin D, or of free and/or bioavailablevitamin D, is compared. Thus, in some embodiments, the methods includedetermining a test subject's VDBP; determining a level of total vitaminD, or of free and/or bioavailable vitamin D in a sample from the testsubject; and comparing the level in the sample to a reference level forthat genotype, e.g., a reference level determined for a subject who hasthe same genotype as the test subject. A reference level for a controlsubject who has the same genotype as the test subject can represent athreshold level below which a subject having that genotype is consideredto have vitamin D insufficiency. Suitable reference levels that aregenotype-specific can be determined using methods known in the art,e.g., including statistically determining an appropriate reference levelin a cohort of subjects who have the same genotype as the test subject.For example, one reference level could be a level of total vitamin D, orof free and/or bioavailable vitamin D, lower than the 25^(th) percentilevalue or 25% of the mean value, of total vitamin D, or of free and/orbioavailable vitamin D, in a population of healthy subjects who have anidentified genotype.

Other suitable reference levels that can be used in a method describedherein include cutoff points for the lowest statistically determinedinterval or portion of the population, e.g., the bottom tertile,quartile, or quintile.

Methods of Treating Vitamin D Insufficiency

Aspects of the invention described herein are directed to methods oftreating a vitamin D insufficiency comprising detecting the level ofbioavailable or free vitamin D in a blood sample obtained from a subjectand administering a treatment for vitamin D insufficiency if the levelof bioavailable vitamin D is lower than a threshold level, e.g., the25th percentile value or 25% of the mean value, of bioavailable vitaminD in a population of healthy subjects. In some embodiments, the level ofbioavailable or free vitamin D is determined by determining the level ofVDBP polypeptide, albumin polypeptide and total vitamin D in a bloodsample obtained from a subject and calculating the level of free and/orbioavailable vitamin D as described above herein. In some embodiments,the level of free (unbound) and/or bioavailable vitamin D can bedetermined directly. In some embodiments, the level of free vitamin Dcan be determined directly and used to calculate the level ofbioavailable vitamin D. In some embodiments, the level of free vitamin Dand the level of albumin polypeptide can be determined directly and usedto calculate the level of bioavailable vitamin D.

In some embodiments, a treatment for vitamin D insufficiency caninclude, for example, compounds which increase the level of vitamin D,bioavailable vitamin D and/or free vitamin D in the subject by providingone or more forms of vitamin D, stimulating the endogenous production ofvitamin D, stimulating the production of active forms of vitamin Dand/or inhibiting the metabolism of vitamin D. Many naturally-occurringforms of vitamin D, derivatives and analogs thereof can be administeredto subjects in need of a vitamin D insufficiency treatment. In someembodiments, any form of vitamin D or a derivative or analog thereof maybe used provided that it exhibits one or more activities ofnaturally-occurring active vitamin D (e.g. increases intestinal calciumabsorption, serum calcium levies or bone mineralization) or ismetabolized to a compound that exhibits such activity. Non-limitingexamples of such compounds include alfacalcidol; calcifediol;calcipotriene; calcidiol; calcitriol (Rocaltrol; Roche);dihydrotachysterol (DH™ and DHT Intensol™; Roxane Laboratories);doxercalciferol (Hectorol®; Genzyme); paricalcitol (Zemplar®; AbbottLaboratories); cholecalciferol (Delta D3™; Freeda Vitamins Inc.) andergocalciferol 3?(Drisdol; Sanofi). Cholecalciferol and ergocalciferolare available as dietary supplements. Further non-limiting examplesinclude 5,6-trans-cholecalciferols; 5,6-trans-ergocalciferols;fluorinated cholecalciferols; side chain homologated cholecalciferols;side chain-truncated cholecalciferols; 19-nor cholecalciferols andergocalciferols; 10,19-dihydovitamin D compounds; 25-hydroxyvitamin D3;25-hydroxyvitamin D2; 24,24-difluoro-25-hydroxyvitamin D3;24-fluoro-25-hydroxyvitamin D3;26,26,26,27,27hexafluoro-25-hydroxyvitaminn D3; 24-25-dihydroxyvitaminD3; d5,26dihydroxyvitamin D3; 23,25,26-trihydroxyvitamin D3;25-hydroxyvitamin D3; the side chain, nor, dinor, trinor andtetranoranalogs of 25-hydroxyvitamin D3, 24-homo-1,25-dihydroxyvitaminD3; 24-dihomo-1,25-dihydroxyvitamin D3; 24-trihomo-1,25-dihydroxyvitaminD3; as well as the corresponding 19-nor compounds of those listed above.

Vitamin D activity can be assayed by a number of methods known in theart. A non-limiting example of an assay to determine if a compound hasvitamin D activity or is metabolized m the subject's body to a compoundhaving vitamin D activity is described in U.S. Pat. No. 5,532,229 whichis incorporated by reference herein in its entirety. Briefly, thecompound is administered and the levels of serum calcium are determinedby chemical colorimetry or by treating with nitric acid and measuringatomic absorption. Administration of a compound having vitamin Dactivity or that is metabolized to a compound having vitamin D activitywill increase the serum calcium levels. Other non-limiting examples ofassays for vitamin D activity include bone mineral density as measuredby x-ray absorptiometry (DEXA) or measurement of serum osteocalcin (seeU.S. Pat. No. 5,972,917 which is incorporated by reference herein in itsentirety).

The dosage of a treatment for vitamin D insufficiency can be determinedby a physician and adjusted, as necessary, to suit observed effects ofthe treatment. With respect to duration and frequency of treatment, itis typical for skilled clinicians to monitor subjects in order todetermine when the treatment is providing therapeutic benefit, and todetermine whether to increase or decrease dosage, increase or decreaseadministration frequency, discontinue treatment, resume treatment ormake other alteration to treatment regimen.

The dosage ranges for the administration of a treatment for vitamin Dinsufficiency depend upon the form of the treatment for vitamin Dinsufficiency, and its potency, as described further herein, and areamounts large enough to produce the desired effect in which thesymptoms, markers, or signs of vitamin D insufficiency are reduced. Insome embodiments, the symptoms, markers, or signs of vitamin Dinsufficiency can include the level of bioavailable or free vitamin Ddetermined according to the methods described herein. The dosage shouldnot be so large as to cause substantial adverse side effects. Generally,the dosage can vary with the age, condition, and sex of the patient andcan be determined by one of ordinary skill in the art. The dosage canalso be adjusted by the individual physician in the event of anycomplication or based upon the subject's sensitivity to the treatment.By way of non-limiting example, forms of vitamin D or a derivative oranalog thereof are typically administered in a therapeutically effectiveamount of from about 0.1 μg to about 2 mg per day depending upon thecompound being administered.

In some embodiments, a vitamin D insufficiency treatment can beadministered over a period of time, such as over a 5 minute, 10 minute,15 minute, 20 minute, or 25 minute period. In some embodiments, theadministration can be repeated, for example, on a regular basis, such ashourly for 3 hours, 6 hours, 12 hours or daily or longer or such asbiweekly (i.e., every two weeks) for one month, two months, threemonths, four months or longer. In some embodiments, when multiple dosesare administered, the doses can be separated from one another by, forexample, six hours, one day, two days, one week, two weeks, one month,or two months.

After an initial treatment regimen, the treatments can be administeredon a less frequent basis. For example, after administration biweekly forthree months, administration can be repeated once per month, for sixmonths or a year or longer. In some embodiments, administration can bechronic, e.g., one or more doses daily over a period of weeks or months.

Administration of a treatment for vitamin D insufficiency can reducelevels of a marker or symptom of vitamin D insufficiency or a disease orcondition associated with vitamin D insufficiency by at least 10%, atleast 15%, at least 20%, at least 25%, at least 30%, at least 40%, atleast 50%, at least 60%, at least 70%, at least 80% or at least 90% ormore. As used herein, the phrase “therapeutically effective amount”,“effective amount” or “effective dose” refers to an amount that providesa therapeutic benefit in the treatment or management of vitamin Dinsufficiency. Vitamin D insufficiency can be determined according tothe methods described herein and a therapeutically effective amount canbe an amount that provides a statistically significant improvement inthe level of bioavailable or free vitamin D as determined according tothe methods described herein. Determination of a therapeuticallyeffective amount is well within the capability of those of ordinaryskill in the art. Generally, a therapeutically effective amount can varywith the subject's history, age, condition, and gender, as well as theseverity and type of the medical condition in the subject, andadministration of other pharmaceutically active agents.

In some embodiments, the administration is repeated until the level ofbioavailable or free vitamin D, as determined according to the methodsdescribed herein, no longer indicates that the subject is vitamin Dinsufficient.

It is to be understood that, for any particular subject, specific dosageregimes should be adjusted over time according to the individual needand the professional judgment of the person administering or supervisingthe administration of the compositions. For example, the dosage of thetherapeutic can be increased if the lower dose does not providesufficient therapeutic activity.

Alternative methods of treating a subject for a vitamin D insufficiencycan include, by way of non-limiting example, exposure to sunlight oradministration of a CYP24 inhibitor (see U.S. patent application Ser.No. 12/935,139); dentonin peptides (see U.S. patent application Ser. No.10/360,202); IL-10 or IL-4 polypeptides or TNF-α inhibitors (see U.S.patent application Ser. No. 10/170,746); or PHEX polypeptides asdescribed in U.S. patent application Ser. No. 10/360,202.

With respect to the therapeutic methods of the invention, unlessotherwise specified, it is not intended that the administration of thevitamin D insufficiency treatment be limited to a particular mode ofadministration, dosage, or frequency of dosing; the present inventioncontemplates all modes of administration, including intramuscular,intravenous, intraperitoneal, intravesicular, intraarticular, topically,subcutaneous, orally or any other route sufficient to provide a doseadequate to treat the vitamin D insufficiency.

Systems

In some aspects, the invention described herein is directed to systems(and computer readable media for causing computer systems) for obtainingdata from at least one blood sample obtained from at least one subject,the system comprising 1) a determination module configured to receivethe at least one blood sample and perform at least one analysis on theat least one blood sample to determine the level of bioavailable or freevitamin D in the sample; 2) a storage device configured to store dataoutput from the determination module; and 3) a display module fordisplaying a content based in part on the data output from thedetermination module, wherein the content comprises a signal indicativeof the level of bioavailable or free vitamin D.

In one embodiment, provided herein is a system comprising: (a) at leastone memory containing at least one computer program adapted to controlthe operation of the computer system to implement a method that includes(i) a determination module configured to receive the at least one bloodsample and perform at least one analysis on the at least one bloodsample to determine the level of bioavailable or free vitamin D in thesample (e.g. determining the level of one or more of VDBP polypeptide,albumin polypeptide, total vitamin D; bioavailable vitamin D; and freevitamin D); (ii) a storage module configured to store output data fromthe determination module; (iii) a computing module adapted to identifyfrom the output data whether the level of VDBP polypeptide, albuminpolypeptide, total vitamin D, bioavailable vitamin D or free vitamin Din a blood sample obtained from a subject indicates that the level ofbioavailable or free vitamin D is lower than a threshold level, e.g.,the 25th percentile value or 25% of the mean value, of bioavailable orfree vitamin D in a population of healthy subjects and (iv) a displaymodule for displaying a content based in part on the data output fromthe determination module, wherein the content comprises a signalindicative of the level of bioavailable or free vitamin D and (b) atleast one processor for executing the computer program (see FIG. 6).

The term “computer” can refer to any non-human apparatus that is capableof accepting a structured input, processing the structured inputaccording to prescribed rules, and producing results of the processingas output. Examples of a computer include: a computer; a general purposecomputer; a supercomputer; a mainframe; a super mini-computer, amini-computer; a workstation; a micro-computer; a server; an interactivetelevision; a hybrid combination of a computer and an interactivetelevision; and application-specific hardware to emulate a computerand/or software. A computer can have a single processor or multipleprocessors, which can operate in parallel and/or not in parallel. Acomputer also refers to two or more computers connected together via anetwork for transmitting or receiving information between the computers.An example of such a computer includes a distributed computer system forprocessing information via computers linked by a network.

The term “computer-readable medium” may refer to any storage device usedfor storing data accessible by a computer, as well as any other meansfor providing access to data by a computer. Examples of astorage-device-type computer-readable medium include: a magnetic harddisk; a floppy disk; an optical disk, such as a CD-ROM and a DVD; amagnetic tape; a memory chip.

The term a “computer system” may refer to a system having a computer,where the computer comprises a computer-readable medium embodyingsoftware to operate the computer.

The term “software” is used interchangeably herein with “program” andrefers to prescribed rules to operate a computer. Examples of softwareinclude: software; code segments; instructions; computer programs; andprogrammed logic.

The computer readable storage media can be any available tangible mediathat can be accessed by a computer. Computer readable storage mediaincludes volatile and nonvolatile, removable and non-removable tangiblemedia implemented in any method or technology for storage of informationsuch as computer readable instructions, data structures, program modulesor other data. Computer readable storage media includes, but is notlimited to, RAM (random access memory), ROM (read only memory), EPROM(erasable programmable read only memory), EEPROM (electrically erasableprogrammable read only memory), flash memory or other memory technology,CD-ROM (compact disc read only memory), DVDs (digital versatile disks)or other optical storage media, magnetic cassettes, magnetic tape,magnetic disk storage or other magnetic storage media, other types ofvolatile and non-volatile memory, and any other tangible medium whichcan be used to store the desired information and which can accessed by acomputer including and any suitable combination of the foregoing.

Computer-readable data embodied on one or more computer-readable mediamay define instructions, for example, as part of one or more programsthat, as a result of being executed by a computer, instruct the computerto perform one or more of the functions described herein, and/or variousembodiments, variations and combinations thereof. Such instructions maybe written in any of a plurality of programming languages, for example.Java, J#, Visual Basic, C, C#, C++, Fortran, Pascal, Eiffel, Basic,COBOL assembly language, and the like, or any of a variety ofcombinations thereof. The computer-readable media on which suchinstructions are embodied may reside on one or more of the components ofeither of a system, or a computer readable storage medium describedherein, may be distributed across one or more of such components.

The computer-readable media may be transportable such that theinstructions stored thereon can be loaded onto any computer resource toimplement the aspects of the present invention discussed herein. Inaddition, it should be appreciated that the instructions stored on thecomputer-readable medium, described above, are not limited toinstructions embodied as part of an application program running on ahost computer. Rather, the instructions may be embodied as any type ofcomputer code (e.g., software or microcode) that can be employed toprogram a computer to implement aspects of the present invention. Thecomputer executable instructions may be written in a suitable computerlanguage or combination of several languages. Basic computationalbiology methods are known to those of ordinary skill in the art and aredescribed in, for example, Setubal and Meidanis et al., Introduction toComputational Biology Methods (PWS Publishing Company, Boston, 1997);Salzberg, Searles, Kasif, (Ed.), Computational Methods in MolecularBiology, (Elsevier, Amsterdam, 1998); Rashidi and Buehler,Bioinformatics Basics: Application in Biological Science and Medicine(CRC Press, London, 2000) and Ouelette and Bzevanis Bioinformatics: APractical Guide for Analysis of Gene and Proteins (Wiley & Sons, Inc.,2nd ed., 2001).

Embodiments of the invention can be described through functionalmodules, which are defined by computer executable instructions recordedon computer readable media and which cause a computer to perform methodsteps when executed. The modules are segregated by function for the sakeof clarity. However, it should be understood that the modules/systemsneed not correspond to discreet blocks of code and the describedfunctions can be carried out by the execution of various code portionsstored on various media and executed at various times. Furthermore, itshould be appreciated that the modules can perform other functions, thusthe modules are not limited to having any particular functions or set offunctions.

The functional modules of certain embodiments of the invention includeat minimum a determination module, a storage module, a computing module,and a display module. The functional modules can be executed on one, ormultiple, computers, or by using one, or multiple, computer networks.The determination module has computer executable instructions to providee.g., levels of expression products etc in computer readable form.

The determination module can comprise any system for detecting a signalelicited from an assay to determine the level of any of a VDBPpolypeptide, an albumin polypeptide, bioavailable vitamin D, freevitamin D, or total vitamin D as described above herein. In someembodiments, such systems can include an instrument, e.g., AU2700(Beckman Coulter Brea, Calif.) as described herein for quantitativemeasurement of polypeptides. In another embodiment, the determinationmodule can comprise multiple units for different functions, such asquantitative measurement of polypeptides (e.g. dye-based photometricassay or quantitative ELISA) and a mass spectroscopy system for themeasurement of vitamin D. In one embodiment, the determination modulecan be configured to perform the methods described elsewhere herein,e.g. dye-based photometric assays for albumin, ELISA assays for VDBPpolypeptide levels or mass spectroscopy to determine vitamin D levels.In some embodiments, such systems can include an instrument, e.g., theAU2700 (Beckman Coulter Brea, Calif.).

In some embodiments, the determination system or a further module can beconfigured to process whole blood samples, e.g. to separate serum fromwhole blood for use in the assays described herein.

The information determined in the determination system can be read bythe storage module. As used herein the “storage module” is intended toinclude any suitable computing or processing apparatus or other deviceconfigured or adapted for storing data or information. Examples ofelectronic apparatus suitable for use with the present invention includestand-alone computing apparatus, data telecommunications networks,including local area networks (LAN), wide area networks (WAN), Internet,Intranet, and Extranet, and local and distributed computer processingsystems. Storage modules also include, but are not limited to: magneticstorage media, such as floppy discs, hard disc storage media, magnetictape, optical storage media such as CD-ROM, DVD, electronic storagemedia such as RAM, ROM, EPROM, EEPROM and the like, general hard disksand hybrids of these categories such as magnetic/optical storage media.The storage module is adapted or configured for having recorded thereon,for example, sample name, biomolecule assayed and the level of saidbiomolecule. Such information may be provided in digital form that canbe transmitted and read electronically, e.g., via the Internet, ondiskette, via USB (universal serial bus) or via any other suitable modeof communication.

As used herein, “stored” refers to a process for encoding information onthe storage module. Those skilled in the art can readily adopt any ofthe presently known methods for recording information on known media togenerate manufactures comprising expression level information.

In some embodiments of any of the systems described herein, the storagemodule stores the output data from the determination module. Inadditional embodiments, the storage module stores reference informationsuch as levels of bioavailable or free vitamin D in healthy subjectsand/or a population of healthy subjects.

The “computing module” can use a variety of available software programsand formats for computing the level of bioavailable or free vitamin D.Such algorithms are well established in the art. A skilled artisan isreadily able to determine the appropriate algorithms based on the sizeand quality of the sample and type of data. The data analysis tools andequations described herein can be implemented in the computing module ofthe invention. In one embodiment, the computing module further comprisesa comparison module, which compares the level of bioavailable or freevitamin D in a blood sample obtained from a subject as described hereinwith the mean value of bioavailable or free vitamin D in a population ofhealthy subjects (FIG. 7). By way of an example, when the value ofbioavailable vitamin D in a blood sample obtained from a subject ismeasured, a comparison module can compare or match the output data—withthe mean value of bioavailable vitamin D in a population of healthysubjects. In certain embodiments, the mean value of bioavailable or freevitamin D in a population of healthy subjects can be pre-stored in thestorage module. During the comparison or matching process, thecomparison module can determine whether the level of bioavailable orfree vitamin D in the blood sample obtained from a subject is lower thana threshold level, e.g., the 25th percentile value or 25% of the meanvalue, of bioavailable or free vitamin D in a population of healthysubjects. In various embodiments, the comparison module can beconfigured using existing commercially-available or freely-availablesoftware for comparison purpose, and may be optimized for particulardata comparisons that are conducted.

The computing and/or comparison module, or any other module of theinvention, can include an operating system (e.g., UNIX) on which runs arelational database management system, a World Wide Web application, anda World Wide Web server. World Wide Web application includes theexecutable code necessary for generation of database language statements(e.g., Structured Query Language (SQL) statements). Generally, theexecutables will include embedded SQL statements. In addition, the WorldWide Web application may include a configuration file which containspointers and addresses to the various software entities that comprisethe server as well as the various external and internal databases whichmust be accessed to service user requests. The Configuration file alsodirects requests for server resources to the appropriate hardware—as maybe necessary should the server be distributed over two or more separatecomputers. In one embodiment, the World Wide Web server supports aTCP/IP protocol. Local networks such as this are sometimes referred toas “Intranets.” An advantage of such Intranets is that they allow easycommunication with public domain databases residing on the World WideWeb (e.g., the GenBank or Swiss Pro World Wide Web site). In someembodiments users can directly access data (via Hypertext links forexample) residing on Internet databases using a HTML interface providedby Web browsers and Web servers (FIG. 8).

The computing and/or comparison module provides a computer readablecomparison result that can be processed in computer readable form bypredefined criteria, or criteria defined by a user, to provide contentbased in part on the comparison result that may be stored and output asrequested by a user using an output module, e.g., a display module.

In some embodiments, the content displayed on the display module can bethe level of bioavailable or free vitamin D in the blood sample obtainedfrom a subject. In some embodiments, the content displayed on thedisplay module can be the relative level of bioavailable or free vitaminD in the blood sample obtained from a subject as compared to the meanlevel of bioavailable or free vitamin D in a population of healthysubjects. In some embodiments, the content displayed on the displaymodule can indicate whether the level of bioavailable or free vitamin Din the blood sample obtained from a subject is less or more than athreshold level, e.g., the 25th percentile value or 25% of the meanvalue, of bioavailable or free vitamin D in a population of healthysubjects. In some embodiments, the content displayed on the displaymodule can indicate whether the subject has an insufficient level ofvitamin D. In some embodiments, the content displayed on the displaymodule can indicate whether the subject is in need of a treatment forvitamin D insufficiency. In some embodiments, the content displayed onthe display module can indicate whether the subject has an increasedrisk or likelihood of having or developing a vitamin D-associateddisease. In some embodiments, the content displayed on the displaymodule can be a numerical value indicating one of these risks orprobabilities. In such embodiments, the probability can be expressed inpercentages or a fraction. For example, higher percentage or a fractioncloser to 1 indicates a higher likelihood of a subject having a vitaminD-associated disease. In some embodiments, the content displayed on thedisplay module can be single word or phrases to qualitatively indicate arisk or probability. For example, a word “unlikely” can be used toindicate a lower risk for having or developing a vitamin D-associateddisease, while “likely” can be used to indicate a high risk for havingor developing a vitamin D-associated disease.

In one embodiment of the invention, the content based on the computingand/or comparison result is displayed on a computer monitor. In oneembodiment of the invention, the content based on the computing and/orcomparison result is displayed through printable media. The displaymodule can be any suitable device configured to receive from a computerand display computer readable information to a user. Non-limitingexamples include, for example, general-purpose computers such as thosebased on Intel PENTIUM-type processor, Motorola PowerPC, Sun UltraSPARC,Hewlett-Packard PA-RISC processors, any of a variety of processorsavailable from Advanced Micro Devices (AMD) of Sunnyvale, Calif., or anyother type of processor, visual display devices such as flat paneldisplays, cathode ray tubes and the like, as well as computer printersof various types.

In one embodiment, a World Wide Web browser is used for providing a userinterface for display of the content based on the computing/comparisonresult. It should be understood that other modules of the invention canbe adapted to have a web browser interface. Through the Web browser, auser can construct requests for retrieving data from thecomputing/comparison module. Thus, the user will typically point andclick to user interface elements such as buttons, pull down menus,scroll bars and the like conventionally employed in graphical userinterfaces.

In some embodiments, the system further comprises a means of inputting avalue for the level of one or more of VDBP polypeptide, albuminpolypeptide, and total vitamin D determined to be in a blood sampleobtained from a subject. By way of non-limiting example, the level ofalbumin polypeptide can be determined by the determination module of thesystem while the level of VDBP polypeptide is determined by an ELISAassay performed separately from the system described herein. When thelevel of VDBP polypeptide is determined, the value for this level can beentered into the computing module of the system and used to determinethe level of bioavailable or free vitamin D in the blood sample obtainedfrom the subject. In some embodiments, the inputting means comprises akeyboard or touchscreen which allows a user to type a value which isaccepted by the computing module.

Systems and computer readable media described herein are merelyillustrative embodiments of the invention for determining the level ofbioavailable or free vitamin D in a blood sample obtained from asubject, and therefore are not intended to limit the scope of theinvention. Variations of the systems and computer readable mediadescribed herein are possible and are intended to fall within the scopeof the invention.

The modules of the machine, or those used in the computer readablemedium, may assume numerous configurations. For example, function may beprovided on a single machine or distributed over multiple machines.

The description of embodiments of the disclosure is not intended to beexhaustive or to limit the disclosure to the precise form disclosed.While specific embodiments of, and examples for, the disclosure aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the disclosure, as thoseskilled in the relevant art will recognize. For example, while methodsteps or functions are presented in a given order, alternativeembodiments can perform functions in a different order, or functions canbe performed substantially concurrently. The teachings of the disclosureprovided herein can be applied to other procedures or methods asappropriate. The various embodiments described herein can be combined toprovide further embodiments. Aspects of the disclosure can be modified,if necessary, to employ the compositions, functions and concepts of theabove references and application to provide yet further embodiments ofthe disclosure. These and other changes can be made to the disclosure inlight of the detailed description.

Specific elements of any of the foregoing embodiments can be combined orsubstituted for elements in other embodiments. Furthermore, whileadvantages associated with certain embodiments of the disclosure havebeen described in the context of these embodiments, other embodimentscan also exhibit such advantages, and not all embodiments neednecessarily exhibit such advantages to fall within the scope of thedisclosure.

All patents and other publications identified are expressly incorporatedherein by reference for the purpose of describing and disclosing, forexample, the methodologies described in such publications that might beused in connection with the present invention. These publications areprovided solely for their disclosure prior to the filing date of thepresent application. Nothing in this regard should be construed as anadmission that the inventors are not entitled to antedate suchdisclosure by virtue of prior invention or for any other reason. Allstatements as to the date or representation as to the contents of thesedocuments is based on the information available to the applicants anddoes not constitute any admission as to the correctness of the dates orcontents of these documents.

Some embodiments of the present invention can be defined as any of thefollowing numbered paragraphs:

-   -   1. An assay comprising:        -   analyzing a blood sample obtained from a subject to            determine a level of VDBP (vitamin D binding protein)            polypeptide, albumin polypeptide and total vitamin D;        -   wherein a level of bioavailable vitamin D is:

=(K _(alb)*[Alb]+1)*[Free Vitamin D]

-   -   -   and wherein a level of free vitamin D is:

=(−{K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1}+√{(K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1)²+4·(K _(DBP) ·K _(alb)·[Alb]+K _(DBP))·([Total VitaminD])}}÷(2·{K _(DBP) ·K _(alb)·[Alb]+K _(DBP)})

-   -   1a. A differential affinity precipitation assay performed by a        method comprising:        -   contacting a sample comprising serum or plasma from a            subject with purified Vitamin D Binding Polypeptide (VDBP),            wherein the purified VDBP is immobilized on a substrate            (e.g., beads, solid surface) for a time sufficient for free            and albumin-bound Vitamin D in the sample to bind to the            purified VDBP, thereby forming a test sample comprising            Vitamin D-VDBP complexes;            -   optionally removing any Vitamin D not bound to the                purified VDBP from the test sample; contacting the                Vitamin D-VDBP complexes with a known amount of free                labeled Vitamin D, for a time sufficient for the labeled                Vitamin D to equilibrate with the Vitamin D-VDBP                complexes in the test sample; and preferably removing                any labeled Vitamin D from the sample that is not bound                to the VDBP;        -   determining the amount of labeled Vitamin D bound to the            purified VDBP in the test sample, and calculating the amount            of bioavailable Vitamin D in the sample from the subject            based on the amount of labeled Vitamin D bound to the            purified VDBP in the test sample.    -   2. The assay of paragraphs 1 or 1a, wherein a level of        bioavailable vitamin D lower than a threshold level, e.g., the        25th percentile value or 25% of the mean value, of bioavailable        vitamin D in a population of healthy subjects indicates that the        subject has a vitamin D insufficiency.    -   3. The assay of any of paragraphs 1-2, wherein the vitamin D is        selected from the group consisting of:        -   25-hydroxyvitamin D and 1,25-dihydroxyvitamin D.    -   4. The assay of any of paragraphs 1-3, wherein the determining        of the level of VDBP polypeptide or albumin polypeptide        comprises use of a method selected from the group consisting of:        -   enzyme linked immunosorbent assay; chemiluminescent            immunosorbent assay; electrochemiluminescent immunosorbent            assay; fluorescent immunosorbent assay; dye linked            immunosorbent assay; immunoturbidimetric assay;            immunonephelometric assay; dye-based photometric assay;            western blot; immunoprecipitation; radioimmunological assay            (RIA); radioimmunometric assay; immunofluorescence assay and            mass spectroscopy.    -   5. The assay of any of paragraphs 1-4, wherein the determining        of the level of total vitamin D comprises the use of a method        selected from the group consisting of:        -   radioimmunoassay; liquid chromatography tandem mass            spectroscopy; enzyme linked immunosorbent assay;            chemiluminescent immunosorbent assay;            electrochemiluminescent immunosorbent assay; fluorescent            immunosorbent assay; and high-pressure liquid            chromatography.    -   6. The assay of any of paragraphs 1-5, wherein an insufficiency        of vitamin D indicates an increased risk of a condition selected        from the group consisting of:        -   decreased bone density; decreased bone mineral density; bone            fractures; bone resorption; rickets; osteitis fibrosa            cystica; fibrogenesis imperfect ossium; osteosclerosis;            osteoporosis; osteomalacia; elevated parathyroid hormone            levels; parathyroid gland hyperplasia secondary            hyperparathyroidism; hypocalcemia; infection; cancer;            psoriasis; cardiovascular disease; renal osteodystrophy;            renal disease; end-stage renal disease; chronic kidney            disease; chronic kidney disease-associated mineral and bone            disorder; extraskeletal calcification; obesity; allergy,            asthama; multiple sclerosis; muscle weakness; rheumatoid            arthritis and diabetes.    -   7. The assay of any of paragraphs 1-6, further comprising the        step of administering a vitamin D insufficiency treatment to a        subject who is determined to have a vitamin D insufficiency.    -   8. The assay of any of paragraphs 1-7, wherein the treatment        comprises administering a compound selected from the group        consisting of:        -   calcitriol; dihydrotachysterol; doxercalciferol;            paricalcitol; cholecalciferol and ergocalciferol.    -   9. An assay comprising:        -   analyzing a blood sample obtained from a subject to            determine a level of VDBP polypeptide, albumin polypeptide            and total vitamin D;        -   wherein a level of free vitamin D is:

={−{K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1}+√{(K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1)²+4·(K _(DBP) ·K _(alb)·[Alb]+K _(DBP))·([Total VitaminD])}}÷(2·{K _(DBP) ·K _(alb)·[Alb]+K _(DBP)})

-   -   10. The assay of paragraph 9, wherein a level of free vitamin D        lower than a threshold level, e.g., the 25th percentile value or        25% of the mean value, of free vitamin D in a population of        healthy subjects indicates that the subject has a vitamin D        insufficiency.    -   11. The assay of any of paragraphs 9-10, wherein the vitamin D        is selected from the group consisting of:        -   25-hydroxyvitamin D and 1,25-dihydroxyvitamin D.    -   12. The assay of any of paragraphs 9-11, wherein the determining        of the level of VDBP polypeptide or albumin polypeptide        comprises the use of a method selected from the group consisting        of:        -   enzyme linked immunosorbent assay; chemiluminescent            immunosorbent assay; electrochemiluminescent immunosorbent            assay; fluorescent immunosorbent assay; dye linked            immunosorbent assay; immunoturbidimetric assay;            immunonephelometric assay; dye-based photometric assay;            western blot; immunoprecipitation; radioimmunological assay            (RIA); radioimmunometric assay; immunofluorescence assay and            mass spectroscopy.    -   13. The assay of any of paragraphs 9-12, wherein the determining        of the level of total vitamin D comprises the use of a method        selected from the group consisting of:        -   radioimmunoassay; liquid chromatography tandem mass            spectroscopy; enzyme linked immunosorbent assay;            chemiluminescent immunosorbent assay;            electrochemiluminescent immunosorbent assay; fluorescent            immunosorbent assay; and high-pressure liquid            chromatography.    -   14. The assay of any of paragraphs 9-13, wherein an        insufficiency of vitamin D indicates an increased risk of a        condition selected from the group consisting of:        -   decreased bone density; decreased bone mineral density; bone            fractures; bone resorption; rickets; osteitis fibrosa            cystica; fibrogenesis imperfect ossium; osteosclerosis;            osteoporosis; osteomalacia; elevated parathyroid hormone            levels; parathyroid gland hyperplasia; secondary            hyperparathyroidism; hypocalcemia; infection; cancer;            psoriasis; cardiovascular disease; renal osteodystrophy;            renal disease; end-stage renal disease; chronic kidney            disease; chronic kidney disease-associated mineral and bone            disorder; extraskeletal calcification; obesity; allergy,            asthama; multiple sclerosis; muscle weakness; rheumatoid            arthritis and diabetes.    -   15. The assay of any of paragraphs 9-14, further comprising the        step of administering a vitamin D insufficiency treatment to a        subject who is determined to have a vitamin D insufficiency.    -   16. The assay of any of paragraphs 9-15, wherein the treatment        comprises administering a compound selected from the group        consisting of:        -   calcitriol; dihydrotachysterol; doxercalciferol;            paricalcitol; cholecalciferol and ergocalciferol.    -   17. A method for treating a vitamin D insufficiency in a subject        comprising detecting a level of VDBP polypeptide, albumin        polypeptide and total vitamin D in a blood sample obtained from        a subject;        -   wherein a level of bioavailable vitamin D is:

=(K _(alb)*[Alb]+1)*[Free Vitamin D]

-   -   -   and wherein a level of free vitamin D is:

={−{K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1}+√{(K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1)²+4·(K _(DBP) ·K _(alb)·[Alb]+K _(DBP))·([Total VitaminD])}}÷(2·{K _(DBP) ·K _(alb)·[Alb]+K _(DBP)}).

-   -   -   and administering a treatment for vitamin D insufficiency to            the subject if the level of bioavailable vitamin D is less            than a threshold level, e.g., the 25th percentile value or            25% of the mean value, of the mean value of bioavailable            vitamin D in a population of healthy subjects.

    -   18. A method for treating a vitamin D insufficiency in a subject        comprising detecting a level of VDBP polypeptide, albumin        polypeptide and total vitamin D in a blood sample obtained from        a subject;        -   wherein a level of free vitamin D is:

={−{K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1}+√{(K _(DBP)·[Total DBP]−K _(DBP)·[Total Vitamin D]+K_(alb)·[Alb]+1)²+4·(K _(DBP) ·K _(alb)·[Alb]+K _(DBP))·([Total VitaminD])}}÷(2·{K _(DBP) ·K _(alb)·[Alb]+K _(DBP)})

-   -   -   and administering a treatment for vitamin D insufficiency to            the subject if the level of free vitamin D is less than a            threshold level, e.g., the 25th percentile value or 25% of            the mean value, of free vitamin D in a population of healthy            subjects.

    -   18a. A method for treating a vitamin D insufficiency in a        subject comprising a level of bioavailable vitamin D in a blood        sample obtained from a subject; wherein a level of bioavailable        vitamin D is determined using a method comprising:        -   contacting a sample comprising serum or plasma from a            subject with purified Vitamin D Binding Polypeptide (VDBP),            wherein the purified VDBP is immobilized on a substrate            (e.g., beads, solid surface) for a time sufficient for free            and albumin-bound Vitamin D in the sample to bind to the            purified VDBP, thereby forming a test sample comprising            Vitamin D-VDBP complexes;        -   optionally removing any Vitamin D not bound to the purified            VDBP from the test sample;        -   contacting the Vitamin D-VDBP complexes with a known amount            of free labeled Vitamin D, for a time sufficient for the            labeled Vitamin D to equilibrate with the Vitamin D-VDBP            complexes in the test sample; and preferably removing any            labeled Vitamin D from the sample that is not bound to the            VDBP;        -   determining the amount of labeled Vitamin D bound to the            purified VDBP in the test sample, and calculating the amount            of bioavailable Vitamin D in the sample from the subject            based on the amount of labeled Vitamin D bound to the            purified VDBP in the test sample,        -   and administering a treatment for vitamin D insufficiency to            the subject if the level of free vitamin D is below a            threshold level, e.g., the 25th percentile value or 25% of            the mean value, of the mean value of free vitamin D in a            population of healthy subjects.

    -   19. A system for obtaining data from at least one blood sample        obtained from at least one subject, the system comprising:        -   a determination module configured to receive the at least            one blood sample and perform at least one analysis on the at            least one blood sample to determine a level of bioavailable            or free vitamin D in the sample;        -   a storage device configured to store data output from said            determination module; and        -   a display module for displaying a content based in part on            the data output from said determination module, wherein the            content comprises a signal indicative of the level of            bioavailable or free vitamin D.

    -   20. The system of paragraph 19, wherein the system further        comprises a means of inputting a value for the level of one or        more of VDBP polypeptide, albumin polypeptide, and total vitamin        D determined to be in a test sample.

    -   21. The system of any of paragraphs 19-20, wherein the content        displayed on said display module further comprises a signal        indicative of the subject having an increased likelihood of a        vitamin D insufficiency if the level of bioavailable or free        vitamin D is determined to be lower than a threshold level,        e.g., the 25th percentile value or 25% of the mean value, of        bioavailable vitamin D in a population of healthy subjects.

    -   22. The system of any of paragraphs 19-21, wherein the content        displayed on said display module further comprises a signal        indicative of the subject being recommended to receive a        treatment for vitamin D insufficiency.

    -   23. A method of treatment comprising:        -   analyzing a blood sample obtained from a subject to            determine a level of free or bioavailable vitamin D;        -   wherein a level of free or bioavailable vitamin D lower than            a threshold level, e.g., the 25th percentile value or 25% of            the mean value, of free or bioavailable vitamin D in a            population of healthy subjects indicates that the subject            has a vitamin D insufficiency; and        -   administering a vitamin D insufficiency treatment to a            subject who is determined to have a vitamin D insufficiency.

    -   24. The method of paragraph 23, wherein the vitamin D is        selected from the group consisting of:        -   25-hydroxyvitamin D and 1,25-dihydroxyvitamin D.

    -   25. The method of any of paragraphs 23-24, wherein the        determining of the level of free or bioavailable vitamin D        comprises use of a method selected from the group consisting of:        immunoassay; two-step immunoassay with antibody capture;        one-step immunoassay with immobilized antibody and competitive        detection; one-step immunoassay with immobilized competitor and        labeled antibody; fluorescence polarization immunoassay;        differential precipitation (immunoprecipitation, affinity        precipitation); immunodepletion; and affinity binding        chromatography;        -   and a method selected from the group consisting of:        -   radioimmunoassay; chemiluminescent immunosorbent assay;            electrochemiluminescent immunosorbent assay; fluorescent            immunosorbent assay; dye linked immunosorbent assay; liquid            chromatography tandem mass spectroscopy and high-pressure            liquid chromatography.

    -   26. The method of any of paragraphs 23-25, wherein an        insufficiency of vitamin D indicates an increased risk of a        condition selected from the group consisting of:        -   decreased bone density; decreased bone mineral density; bone            fractures; bone resorption; rickets; osteitis fibrosa            cystica; fibrogenesis imperfect ossium; osteosclerosis;            osteoporosis; osteomalacia; elevated parathyroid hormone            levels; parathyroid gland hyperplasia; secondary            hyperparathyroidism; hypocalcemia; infection; cancer;            psoriasis; cardiovascular disease; renal osteodystrophy;            renal disease; end-stage renal disease; chronic kidney            disease; chronic kidney disease-associated mineral and bone            disorder, extraskeletal calcification; obesity; allergy,            asthama; multiple sclerosis; muscle weakness; rheumatoid            arthritis and diabetes.

    -   27. The method of any of paragraphs 23-26, wherein the treatment        comprises administering a compound selected from the group        consisting of:        -   calcitriol; dihydrotachysterol; doxercalciferol;            paricalcitol; cholecalciferol and ergocalciferol.

    -   28. An assay comprising:        -   analyzing a blood sample obtained from a subject to            determine a level of free vitamin D and albumin polypeptide;        -   wherein a level of bioavailable vitamin D is:

=(K _(alb)*[Alb]+1)*[Free Vitamin D]

-   -   29. The assay of paragraph 28, wherein a level of bioavailable        vitamin D lower than a threshold level, e.g., the 25th        percentile value or 25% of the mean value, of bioavailable        vitamin D in a population of healthy subjects indicates that the        subject has a vitamin D insufficiency.    -   30. The assay of any of paragraphs 28-29, wherein the vitamin D        is selected from the group consisting of:        -   25-hydroxyvitamin D and 1,25-dihydroxyvitamin D.    -   31. The assay of any of paragraphs 28-30, wherein the        determining of the level of albumin polypeptide comprises use of        a method selected from the group consisting of:        -   enzyme linked immunosorbent assay; chemiluminescent            immunosorbent assay; electrochemiluminescent immunosorbent            assay; fluorescent immunosorbent assay; dye linked            immunosorbent assay; immunoturbidimetric assay;            immunonephelometric assay; dye-based photometric assay;            western blot; immunoprecipitation; radioimmunological assay            (RIA); radioimmunometric assay; immunofluorescence assay and            mass spectroscopy.    -   32. The assay of any of paragraphs 28-31, wherein the        determining of the level of free vitamin D comprises the use of        a method selected from the group consisting of:        -   immunoassay; two-step immunoassay with antibody capture;            one-step immunoassay with immobilized antibody and            competitive detection; one-step immunoassay with immobilized            competitor and labeled antibody; fluorescence polarization            immunoassay; differential precipitation            (immunoprecipitation, affinity precipitation);            immunodepletion; and affinity binding chromatography;        -   and a method selected from the group consisting of:        -   radioimmunoassay; chemiluminescent immunosorbent assay;            electrochemiluminescent immunosorbent assay; fluorescent            immunosorbent assay; liquid chromatography tandem mass            spectroscopy and high-pressure liquid chromatography.    -   33. The assay of any of paragraphs 28-32, wherein an        insufficiency of vitamin D indicates an increased risk of a        condition selected from the group consisting of:        -   decreased bone density; decreased bone mineral density; bone            fractures; bone resorption; rickets; osteitis fibrosa            cystica; fibrogenesis imperfect ossium; osteosclerosis;            osteoporosis; osteomalacia; elevated parathyroid hormone            levels; parathyroid gland hyperplasia; secondary            hyperparathyroidism; hypocalcemia; infection; cancer;            psoriasis; cardiovascular disease; renal osteodystrophy;            renal disease; end-stage renal disease; chronic kidney            disease; chronic kidney disease-associated mineral and bone            disorder; extraskeletal calcification; obesity; allergy,            asthama; multiple sclerosis; muscle weakness; rheumatoid            arthritis and diabetes.    -   34. The assay of any of paragraphs 28-33, further comprising the        step of administering a vitamin D insufficiency treatment to a        subject who is determined to have a vitamin D insufficiency.    -   35. The assay of any of paragraphs 28-34, wherein the treatment        comprises administering a compound selected from the group        consisting of:        -   calcitriol; dihydrotachysterol; doxercalciferol;            paricalcitol; cholecalciferol and ergocalciferol.

EXAMPLES

This invention is further illustrated by the following examples whichshould not be construed as limiting.

Example 1 Bioavailable Vitamin D and Bone Mineral Density

Studies examining the relationship between total circulating25-hydroxyvitamin D (25(OH)D) levels and bone mineral density (BMD) haveyielded mixed results. Vitamin D binding protein (DBP), the majorcarrier protein for 25(OH)D, may alter the biologic activity ofcirculating vitamin D. Demonstrated herein is a test of the hypothesisthat free and bioavailable 25(OH)D, calculated from total 25(OH)D, DBPand serum albumin levels, is more strongly associated with BMD thanlevels of total 25(OH)D.

Total 25(OH)D, DBP, and serum albumin levels were measured in 49 healthyyoung adults enrolled in the Metabolic Abnormalities in College-AgedStudents (MACS) study. Lumbar spine BMD was measured in all subjectsusing dual X-ray absorptiometry. Clinical, diet, and laboratoryinformation was also gathered at this time. Free and bioavailable(free+albumin bound) 25(OH)D was determined and their associations withBMD were examined.

1 BMD was not associated with total 25(OH)D levels (r=0.172 p=0.236). Incontrast, free and bioavailable 25(OH)D levels and the ratio of total25(OH)D concentrations divided by VDBP concentrations (referred toherein as the Vitamin D Index) were positively correlated with BMD(r=0.413 p=0.003 for free, r=0.441 p=0.002 for bioavailable, r=0.406p=0.003). Bioavailable 25(OH)D levels remained independently associatedwith BMD in multivariate regression models adjusting for age, sex, bodymass index, and race (p=0.03). Free and bioavailable 25(OH)D are morestrongly correlated with BMD than total 25(OH)D. These findings haveimportant implications for vitamin D supplementation in vitamin Ddeficient states.

Introduction

Vitamin D insufficiency is associated with decreased calcium absorptionand elevated levels of parathyroid hormone (PTH),(1) which may lead toexcessive bone resorption.(2) In some observational studies, higherlevels of 25-hydroxyvitamin D (25(OH)D) have been linked to increasedbone mineral density (BMD) and decreased risk of fracture.(3-7)Additionally, several randomized control trials suggest that vitamin Dsupplementation reduces the risk of fracture and increases BMD.(8-14)However, not all observational studies have confirmed the relationshipbetween 25-hydroxyvitamin D (25(OH)D) and BMD, especially in youngerpopulations or racial minorities.(15-19) Moreover, in several randomizedtrials, the effect of vitamin D supplementation on BMD or fracture riskhas been modest,(8) absent,(20-22) or reversed.(23)

The free hormone hypothesis postulates that only hormones liberated frombinding proteins enter cells and produce biological action.(24) 25(OH)Dand 1,25-dihydroxyvitamin D (1,25(OH)2D) circulate bound to vitamin Dbinding protein (85-90%) and albumin (10-15%) with less than 1% ofcirculating hormone in its free form.(25) In mice, vitamin D bindingprotein (DBP) prolongs the serum half life of 25(OH)D and protectsagainst vitamin D insufficiency by serving as a vitamin D reservoir.(26)However, DBP also limits the biological activity of injected 1,2 (OH)2Din mice(26) and inhibits the action of vitamin D on monocytes andkeratinocytes in vitro.(27-28). The significance of circulating DBPlevels with regards to vitamin D's biological action in humans isunclear.

The free fraction of 25(OH)D and the binding affinity constants for25(OH)D's interaction with DBP and albumin have previously beenmeasured.(29) Formulae for the calculation of free 25(OH)D levels basedon serum concentrations of total 25(OH)D, DBP, albumin, and have beendeveloped based on this data. Measured and calculated values of free25(OH)D are highly correlated.(29)

Materials and Methods

Subject Recruitment.

A cross-sectional study was conducted in a subset of healthy youngadults enrolled in the Metabolic Abnormalities in College Students study(MACS), a study designed to evaluate the prevalence of metabolicabnormalities in university students.(30) Subjects were healthy 18-31year old male and female students from private universities in theBoston area. 170 subjects were recruited through flyers postedthroughout the Massachusetts Institute of Technology (MIT) campus andthrough targeted emails to random members of the student population. Allsubjects provided written informed consent. The study was approved bythe MIT Committee on the Use of Humans as Experimental Subjects. 49subjects had sufficient sample for inclusion in this analysis and theircharacteristics are presented in Table 1.

Study Visit.

Subjects were instructed to fast for 12 hours prior to admission to theMIT Clinical Research Center as outpatients and underwent a baselineevaluation including a blood sample collection and various physiologicmeasurements. Structured interviews were conducted by study nurses tocollect standard clinical information, minutes of exercise per week (in30 minute increments), and medication/supplement use. Height wasmeasured using a standing stadiometer (Holton Ltd, Crymych, Dyfed, UK).Weight was measured using a calibrated scale (SECA, Hanover, Md., USA).Body mass index (BMI) was calculated as weight(kg)/[height(m)].

Dietary Information.

Subjects completed a written food record 1 week prior to the day ofstudy, recording 4 full days of food intake, including one weekend day.During the study visit to the MIT CRC, a registered dietitian reviewedthe food record with the subject to clarify the quantities and sourcesof food consumed. Dietary intake data were then analyzed using NutritionData System for Research software version 2006/2007 (NutritionCoordinating Center, University of Minnesota. Minneapolis, Minn.).

Bone Density Measurement.

Subjects underwent total-body dual-energy x-ray absorptiometry (DEXA)(Hologic QDR-4500A; Hologic. Waltham, Mass., USA) to determine total andregional BMD.(31) Hologic phantoms were used to calibrate theinstrument. Lumbar spine BMD was used in this study as the measure ofBMD. Lumbar spine BMD is a preferred site for the diagnosis ofosteoporosis and the prediction of fracture. No hip BMD measurementswere available. (32-33)

Biochemical Analysis.

Baseline blood samples were frozen at −80° C. and stored for lateranalysis. 25(OH)D, serum calcium, albumin, and levels of PTH weremeasured in the Massachusetts General Hospital (MGH) clinicallaboratories. 25(OH)D2 and 25(OH)D3 levels were measured by liquidchromatography tandem mass spectrometry (LC-MS), with interassay CV's of9.1% for 25(OH)D₂ and 8.6% for 25(OH)D₃. Total 25(OH)D level wascalculated as the sum of 25(OH)D₂ level and 25(OH)D₃ level. Intact PTHwas measured by electrochemiluminescense immunoassay on the Cobas E160automated analyzer (Roche Diagnostics, Indianapolis, Ind.). InterassayCV for intact PTH measurement was 4.2%. Calcium and albumin levels weremeasured by dye-based photometric assays on an automated analyzer. DBPwas measured in duplicate by commercial enzyme linked immunosorbentassay (ELISA) (R&D Systems, Minneapolis, Minn., Catalog Number DVDBP0)according to the manufacturer's instructions. The assay was conductedafter diluting serum samples 1 to 2,000 in Calibrator Diluent RD6-11(R&D Systems Part Number 895489). Inter-assay CV was 8.5% at aconcentration of 40 ug/ml. The assay recovered between 93 and 110% of a100-200 ug/mL dose of exogenous vitamin D binding protein added to humanserum samples containing between 25-200 ug/mL of endogenous vitamin Dbinding protein. The manufacturer reports no significantcross-reactivity with human albumin, vitamin D3, or alpha-fetoprotein.In a subset of patients in whom adequate serum was available (N=45),total 1,25(OH)₂D was measured by LC-MS/MS in the Mayo Clinic MedicalLaboratories (Rochester, Minn., USA).

Calculation of Unbound 25(OH)D.

Free levels of 25(OH)D were calculated using two methods. Both methodsused the binding affinity constants between albumin and DBP and 25(OH)Dmeasured in a previous study which used centrifugal ultrafiltration todetermine the free fraction of 25(OH)D.(29)

Method 1:

Free levels of 25(OH)D were calculated using the following equation:

${{Free}\mspace{14mu} 25({OH})D} = {\frac{{Total}\mspace{14mu} 25({OH})D}{1 + \left( {6 \times 10^{5} \times {Albumin}} \right) + \left( {7 \times 10^{8} \times {DBP}} \right)}.}$

(29) The reported correlation coefficient between calculated free25(OH)D using this equation and measured free 25(OH)D by centrifugalultrafiltration is 0.925.(29) Free 1,25(OH)D levels were also calculatedusing this method.(25)

Method 2:

Free, bioavailable, and DBP-bound 25(OH)D were calculated usingequations described in Appendix 1 of this Example below. These methodsdefine bioavailable hormone as the fraction that is both free andalbumin-bound, i.e. the fraction not bound to circulating bindingproteins such as DBP.

Both calculation methods used the same affinity binding constants.Applied to the same measurements of total 25(OH)D, DBP, and albumin,they produce calculated free 25(OH)D values that are highly correlated(Spearman r=1), however the equations produce values that are an averageof 1.4% higher (data not shown). Because the equations also provide forseparate calculation of free, bioavailable, and DBP-bound 25(OH)D,Method 2 procedures were used for subsequent analyses of 25(OH)D levels.

Statistical Analysis.

Subject characteristics are reported as mean±SD unless otherwise noted.Non-normal variables including 25(OH)D levels, DBP levels, BMD, anddietary calcium intake levels showed skewed distributions and werenatural log transformed in order to meet the assumptions of parametricstatistical techniques. Exercise amount was dichotomized at 120 minutesper week. Pearson's correlation coefficients were calculated to assessthe relationships between 25(OH)D levels, BMD, and other continuousvariables. Independent samples t-tests were used to compare 25(OH)Dlevels, DBP levels, and BMD among subgroups defined by race, sex,exercise amount, and oral contraceptive use. Linear regression analysiswas used to test for the presence of an independent relationship between25(OH)D levels, DBP, and BMD after adjustment for factors previouslyreported to be associated with bone density including age, sex, BMI andrace. (5,8,12,35-36) All analyses were conducted using STATA StatisticalSoftware (College Station, Tex.) version 11. Two sided p-values<0.05were considered statistically significant.

Results

Subject characteristics are shown in Table 1. There was wide variationin levels of DBP, with concentrations ranging from 0.66 to 11.2 umol/L.Accordingly, calculated free and bioavailable 25(OH)D levels rangedwidely (Table 2). Total 25-hydroxyvitamin D levels were positivelycorrelated with DBP levels (r=0.335, p=0.019).

Total 25(OH)D levels were not correlated with BMD (r=−0.172 p=0.236,FIG. 1). Similarly, levels of DBP-bound 25(OH)D were not correlated withBMD (r-=0.072, p=0.626). In contrast, free and bioavailable 25(OH)Dlevels and Vitamin D index values were all strongly correlated with BMD(r=0.413 p=0.003 for free, r=0.441 p=0.002 for bioavailable, and r=0.406p=0.003 for Vitamin D Index values, FIG. 1). Bioavailable and free25(OH)D levels were highly correlated with each other (r=0.985,p<0.001), but bioavailable 25(OH)D made up a larger portion of the total25(OH)D with approximately 350-fold higher concentrations ofbioavailable 25(OH)D compared to free 25(OH)D. (Table 2) Total andcalculated free levels of 1,25(OH)₂D were not correlated with BMD(p>0.05). Total levels of 1,25(OH)₂D were not associated with free orbioavailable 25(OH)D levels (p>0.05), nor was sex-adjusted alkalinephosphatase associated with total, free, or bioavailable 25(OH) D(p>0.05) Neither total nor bioavailable 25(OH)D levels were correlatedwith serum calcium or PTH levels (p>0.05). Of note, PTH levels fellbetween 15 and 51 ng/L (all within the normal range) and were notassociated with BMD (r=−0.024, p=0.869).

Both total 25(OH)D and DBP levels were inversely associated with BMI(r=−0.300, p=0.036 for total and r=−0.542, p<0.001 for DBP).Bioavailable 25(OH)D was positively correlated with BMI (r=0.302,p=0.035). However, in this population, BMI was not correlated with BMD(r=0.160, p=0.271). Dietary calcium intake was correlated with total25(OH)D (r=0.339, p=0.021), but was not correlated with DBP,bioavailable 25(OH)D, or BMD (p>0.05). Levels of total and bioavailable25(OH)D, DBP and BMD among selected subgroups are shown in Table 3.Females had greater average total 25(OH)D levels than males, but averageDBP levels, bioavailable 25(OH)D levels and BMD did not differ betweenmales and females. Females reporting use of oral contraceptive pills(OCP) had higher average total 25(OH)D compared with females who did notreport OCP use, but average DBP and bioavailable 25(OH)D levels were notsignificantly different based on OCP use. Subjects with BMI greater thanor equal to 25 kg/m² (overweight subjects) had lower DBP levels thansubjects with BMI less than 25 kg/m² (normal weight subjects). Subjectswho reported exercising 120 minutes a week or more had higher averagetotal 25(OH)D levels than subjects who did not, but no significantdifference was found in average DBP levels, bioavailable 25(OH)D levels,and BMD. Average DBP levels in non-white subjects were lower than inwhite subjects (Table 3).

In multivariate models adjusting for age, sex, BMI and race,bioavailable 25(OH)D remained independently associated with BMD(p=0.03,Table 4). Bioavailable 25(OH)D was the only significant predictor of BMDin multivariate models. As the level of calculated bioavailable 25(OH)Dis dependent on the concentrations of total 25(OH)D, albumin, and DBP,it was separately assessed whether albumin or DBP was associated withBMD. DBP level was inversely correlated with BMD (r=−0.296, p=0.039)while serum albumin showed no association with BMD (r=0.156, p=0.285).In a multivariate linear regression model, total 25(OH)D became asignificant predictor of BMD only after adjustment for DBP level(B=0.089, p=0.040). Albumin was not associated with BMD in amultivariate model including DBP and total 25(OH)D (p=0.150).

Discussion

In light of conflicting reports concerning the relationship betweencirculating levels of 25(OH)D and BMD, serum levels of total 25(OH)D,DBP, and albumin were measured in a group of young healthy adults andassessed relationships between free 25(OH)D, bioavailable 25(OH)D, total25(OH)D and BMD. Without meaning to be limiting, the results describedherein are consistent with the free hormone hypothesis and suggest thatcirculating DBP is an inhibitor of the biological action of vitamin D inhumans. The similar associations between free and bioavailable vitaminlevels and BMD imply that, unlike binding to DBP, binding to albumindoes not inhibit the action of 25(OH)D. These results are consistentwith prior basic and clinical studies on DBP.

The results described herein support the hypothesis that DBP behavessimilarly to other serum hormone carrier proteins and have broadclinical applications. Like thyroid hormone binding globulin and sexhormone binding globulin, DBP may act as a serum carrier and reservoir,prolonging the circulating half-life of vitamin D, while at the sametime regulating its immediate bioavailability to target tissues.(24). Incontrast to the megalin-mediated endocytosis described in renal tubularcells, our results imply that 25(OH)D gains access to some target cellsby diffusion across cell membranes, similar to these other steroidhormones.(24) Thus hormonal activity and sufficiency may be reflected bythe amounts of bioavailable vitamin, not by total serum levels.Currently, clinical testing for vitamin D insufficiency is based uponmeasurement of total serum concentrations of 25(OH)D.(2) Yet, the datadescribed herein suggests that concentrations of total serum vitamin Dmay not be the best measure of vitamin D sufficiency. For example,patients with high levels of DBP may appear to be 25(OH)D sufficient,but may actually be deficient in bioavailable vitamin. Conversely, inpatients with low levels of DBP, total 25(OH)D will be low, but thesepatients may actually have sufficient bioavailable vitamin. Themaintenance of bioavailable 25(OH)D levels in obese and non-whitesubjects, despite lower levels of total 25(OH)D raise the possibilitythat variation in circulating DBP explains the apparent paradox of low25(OH)D levels and higher BMD in black and overweight patients seen inseveral previous studies, (5,15-16,35,44-45).

The results described herein contrast with results of some prior studieslinking total 25(OH)D levels to BMD, but are consistent with otherstudies which failed to find such a relationship.(4-5,15-18) In theseprior studies, DBP levels were not measured. Of note, total 25(OH)D andfree/bioavailable 25(OH)D levels are associated, and it is possible thata larger sample size would have enabled detection of a weak relationshipbetween total 25(OH)D and BMD. Prior studies that found thisrelationship generally had sample sizes greater than 200 and whencorrelation coefficients between 25(OH)D and BMD were reported, theywere less than 0.2.(4-5,16,19)

A relationship between 1,25(OH)₂D levels and BMD was not found. While1,25(OH)₂D is thought to be the active form of vitamin D, many tissuesexpress 1-α hydroxylase, and may be able to convert circulating 25(OH)Dto its active form locally. (46) Circulating 25(OH)D levels aregenerally considered to better reflect overall vitamin D stores. (2) Theresults describe herein are in agreement with this, suggesting thattotal circulating total or free 1,25(OH)₂D levels are not good measuresof vitamin D activity. This is analogous to the accepted model for themeasurement of thyroid hormone action, where free T4 levels are a bettermeasure of thyroid hormone action than circulating free T3 levels, eventhough T3 is the active form of the hormone.(47)

The use of standardized immunoassays for vitamin D, DBP, and albumincombined with standard calculation methods would allow the approachdescribed herein to be adopted with more confidence by other clinicallaboratories.

A wide distribution of DBP levels among our subjects and observed thatDBP was negatively associated with both high BMI and black race, both ofwhich have been associated with low 25(OH)D levels. Without wishing tobe limiting, one potential explanation is that 25(OH)D might itselfregulate the production of DBP. Lowering DBP levels would allow a higherfraction of DBP to be bioavailable in situations where total levels arelow. Other possible explanations for the observed associations betweenrace, BMI, and DBP levels include genetic factors and uptake ofcirculating DBP by adipose tissue.

Described herein is evidence that DBP modifies the relationship between25(OH)D and BMD in humans. Our data suggest that bioavailable 25(OH)Dlevels are a better of measure of vitamin D activity than total 25(OH)Dlevels, at minimum, with respect to bone metabolism. It is thereforepossible that by using total 25(OH)D levels as a measure of vitamin Dsufficiency, individuals may be misclassified as vitamin D sufficient orinsufficient. This may explain conflicting results of prior studies ofthe relationship between serum 25(OH)D concentrations and BMD.Determining which individuals have a true deficit in vitamin D may allowfuture vitamin D supplementation interventions to be targeted to thoseindividuals most likely to benefit. Additionally, use of bioavailable25(OH)D levels may further elucidate the nature of the relationshipbetween vitamin D and a wide range of outcomes including fracture,(7)infection,(49) cancer.(50) and cardiovascular disease.(51)

Appendix. Derivation of Calculated Free and Bioavailable25-Hydroxyvitamin D.

Definitions

D=25-hydroxyvitamin D (calcidiol), sum of both D2 and D3

Alb=albumin

DBP=Vitamin D binding protein, also known as Group-specific component orGc; also referred to herein as VDBP

[D_(Alb)]=concentration of albumin-bound vitamin D

[D_(DBP)]=concentration of DBP-bound vitamin D

[D]=concentration of free (unbound) D

[Total]=concentration of Total 25OH-D=[D_(DBP)]+[D_(Alb)]+[D]

[Bio]=concentration of Bioavailable D (Bioavailable=sum of free andalbumin-bound vitamin)=[D]+[D_(Alb)]

K_(alb)=affinity constant between vitamin D and albumin=6×10⁵ M⁻¹

K_(DBP)=affinity constant between vitamin D and DBP=0.7×10⁹ M⁻¹

Vitamin D Index=ratio of measured concentrations of total25-hydroxyvitamin D and VDBP

Equations

Total 25(OH)-Vitamin D

[Total]=sum of concentrations of 25(OH)-Vitamin D3 and 25(OH)-Vitamin D2

Given that [Total]=[D]+[D _(Alb) ]+[D _(DBP)]

thus [D _(DBP)]=[Total]−[D _(Alb) ]−[D]  (Eq. 1)

Albumin

[Alb]=serum albumin concentration in g/L÷66,430 g/mole

[D]+[Alb]

[D _(Alb)]

Albumin association constant K _(alb) =[D _(Alb)]÷([D]·[Alb])

Thus [D _(Alb) ]=K _(alb)·[Alb]·[D]  (Eq. 2)

-   -   (NB: [Alb] in this example denotes the concentration of free        non-vitamin bound albumin. However, given the low affinity        between albumin and Vit. D, the concentrations of total albumin        and unbound albumin are effectively equivalent ([Total        Albumin]≈[Alb]). As a result, [Alb] may be estimated accurately        by measurements of total serum albumin.)

DBP

[Total DBP]=concentration of serum DBP in g/L÷58,000 g/mole

[DBP]=free unbound DBP and [D _(DBP)]=vitamin-bound DBP

Given that [D]+[DBP]

[D _(DBP)]

And DBP association constant K _(DBP) =[D _(DBP)]÷([DBP]·[D])

Thus [D]=[D _(DBP) ]÷K _(DBP)÷[DBP]  (Eq. 3)

Since [Total DBP]=sum of bound and unbound DBP=[DBP]+[D _(DBP)]

Therefore [DBP]=[Total DBP]−[D _(DBP)]  (Eq. 4)

Solving for Free 25(OH)-Vitamin D

From Eqs. 3 and 4 we see that:

[D]=[D _(DBP) ]÷K _(DBP)÷([Total DBP]−[D _(DBP)])  (Eq. 5)

If we substitute Eq. 1 into Eq. 2, we find that:

[D _(DBP)]=[Total]−(K _(alb)·[Alb]+1)·[D]  (Eq. 6)

Substituting Eq. 6 into Eq. 5 produces the following:

[D]={[Total]−(K _(alb)·[Alb]+1)·[D]}÷K _(DBP)÷([Total DBP]−{[Total]−(K_(alb)·[Alb]+1)·[D]})

The equation is now limited to known constants (K_(DBP) and K_(alb)),measured values ([Total DBP], [Alb], and [Total]) and the dependentvariable for free vitamin D [D]. After propagating products and severalrearrangements we can further simplify this to fit the form of asecond-degree polynomial:

ax ² +bx+c=0

Where x=[D]=the concentration of free 25(OH)-Vitamin D

a=K _(DBP) ·K _(alb)·[Alb]+K _(DBP)

b=K _(DBP)·[Total DBP]−K _(DBP)·[Total]+K _(alb)·[Alb]+1

c=−[Total]

This polynomial may be solved for [D] using the quadratic equation:

[D]=[−b+√b2−4ac]÷2a

-   -   After solving for free 25(011)-vitamin D, we may then use Eq. 2        to calculate the concentration of bioavailable (non-DBP bound        vitamin):

[Bio]=[D]+[D _(Alb)]=(K _(alb)·[Alb]+1)·[D]  (Eq. 7)

Vitamin D Index=[Total]/[Total DBP]   (Eq. 8)

Example Calculation

Total 25(OH)-vitamin D=[Total]=40 ng/mL=1.0×10⁻⁷ mol/L

Total serum DBP=[Total DBP]=250 ug/mL=4.3×10⁻⁶ mol/L

Total serum albumin=[Alb]=4.3 g/dL=6.4×10⁻⁴ mol/L

K _(alb)=6×10⁵M⁻¹

K _(DBP)=7.0×10⁸M⁻¹

a=2.7×10¹¹

b=3325

c=−1×10⁻⁷

Calculated concentration of free 25(OH)D=3.01×10⁻¹¹ mol/L=12.1 pg/mL

Calculated concentration of bioavailable 25(OH)D=1.09×10⁻⁸ mol/L=4.6ng/mL

REFERENCES

-   1. Thomas M K, Lloyd-Jones D M. Thadhani R I, Shaw A C, Deraska D J,    Kitch B T. Vamvakas E C, Dick I M, Prince R L, Finkelstein J S 1998    Hypovitaminosis D in medical inpatients. N Engl J Med    338(12):777-83.-   2. Holick M F 2007 Vitamin D deficiency. N Engl J Med 357(3):266-81.-   3. Bischoff-Ferrari H A. Zhang Y, Kiel D P. Felson D T 2005 Positive    association between serum 25-hydroxyvitamin D level and bone density    in osteoarthritis. Arthritis Rheum 53(6):821-6.-   4. Valimaki V V, Alfthan H, Lehmuskallio E, Loyttyniemi E, Sahi T,    Stenman U H, Suominen H, Valimaki M J 2004 Vitamin D status as a    determinant of peak bone mass in young Finnish men. J Clin    Endocrinol Metab 89(1):76-80.-   5. Bischoff-Ferrari H A. Dietrich T. Orav E J, Dawson-Hughes B 2004    Positive association between 25-hydroxy vitamin D levels and bone    mineral density: a population-based study of younger and older    adults. Am J Med 116(9):634-9.-   6. Stone K, Bauer D C, Black D M, Sklarin P, Ensrud K E, Cummings S    R 1998 Hormonal predictors of bone loss in elderly women: a    prospective study. The Study of Osteoporotic Fractures Research    Group. J Bone Miner Res 13(7):1167-74.-   7. Cauley J A, Lacroix A Z, Wu L, HIorwitz M, Danielson M E, Bauer D    C, Lee J S, Jackson R D, Robbins J A, Wu C, Stanczyk F Z, LeBoffMS,    Wactawski-Wende J, Sarto G, Ockene J, Cummings S R 2008 Serum    25-hydroxyvitamin D concentrations and risk for hip fractures. Ann    Intern Med 149(4):242-50.-   8. Jackson R D, LaCroix A Z, Gass M, Wallace R B, Robbins J, Lewis C    E, Bassford T, Beresford S A, Black H R, Blanchette P, Bonds D E,    Brunner R L, Brzyski R G, Caan B, Cauley J A, Chlebowski R T,    Cummings S R, Granek I, IHays J, Ileiss G, Hlendrix S L, Howard B V,    Hsia J, Hubbell F A, Johnson K C, Judd H, Kotchen J M, Kuller L H,    Langer R D, Lasser N L. Limacher M C. Ludlam S. Manson J E, Margolis    K L, McGowan J, Ockene J K, O'Sullivan M J, Phillips L, Prentice R    L, Sarto G E, Stefanick M L, Van Horn L, Wactawski-Wende J, Whitlock    E, Anderson G L, Assaf A R, Barad D 2006 Calcium plus vitamin D    supplementation and the risk of fractures. N Engl J Med    354(7):669-83.-   9. Dawson-Hlughes B, Harris S S, Krall E A, Dallal G E, Falconer G,    Green C L 1995 Rates of bone loss in postmenopausal women randomly    assigned to one of two dosages of vitamin D. Am J Clin Nutr    61(5):1140-5.-   10. Bischoff-Ferrari H A, Willett W C, Wong J B, Giovannucci E.    Dietrich T, Dawson-Hughes B 2005 Fracture prevention with vitamin D    supplementation: a meta-analysis of randomized controlled trials.    JAMA 293(18):2257-64.-   11. Dawson-Hughes B. Harris S S, Krall E A, Dallal G E 1997 Effect    of calcium and vitamin D supplementation on bone density in men and    women 65 years of age or older. N Engl J Med 337(10):670-6.-   12. Chapuy M C, Arlot M E, Duboeuf F, Brun J, Crouzet B, Arnaud S,    Delmas P D, Meunier P J 1992 Vitamin D3 and calcium to prevent hip    fractures in the elderly women. N Engl J Med 327(23):1637-42.-   13. Ooms M E, Roos J C, Bezemer P D, van der Vijgh W J, Bouter L M,    Lips P 1995 Prevention of bone loss by vitamin D supplementation in    elderly women: a randomized double-blind trial. J Clin Endocrinol    Metab 80(4):1052-8.-   14. Bischoff-Ferrari H A, Willett W C, Wong J B, Stuck A E,    Staehelin H B, Orav E J, Thoma A, Kiel D P, Henschkowski J 2009    Prevention ofnonvertebral fractures with oral vitamin D and dose    dependency: a meta-analysis of randomized controlled trials. Arch    Intern Med 169(6):551-61.-   15. Kremer R, Campbell P P, Reinhardt T. Gilsanz V 2009 Vitamin D    status and its relationship to body fat, final height, and peak bone    mass in young women. J Clin Endocrinol Metab 94(1):67-73.-   16. Hannan M T, Litman H J, Araujo A B, McLennan C E, McLean R R,    McKinlay J B, Chen T C, Holick M F 2008 Serum 25-hydroxyvitamin D    and bone mineral density in a racially and ethnically diverse group    of men. J Clin Endocrinol Metab 93(1):40-6.-   17. Sherman S S, Tobin J D, Hollis B W, Gundberg C M, Roy T A, Plato    C C 1992 Biochemical parameters associated with low bone density in    healthy men and women. J Bone Miner Res 7(10):1123-30.-   18. Gerdhem P. Ringsberg K A, Obrant K J, Akesson K 2005 Association    between 25-hydroxy vitamin D levels, physical activity, muscle    strength and fractures in the prospective population-based OPRA    Study of Elderly Women. Osteoporos Int 16(11):1425-31.-   19. Gutierrez O M, Farwell W R, Kermah D, Taylor E N 2010 Racial    differences in the relationship between vitamin D, bone mineral    density, and parathyroid hormone in the National Health and    Nutrition Examination Survey. Osteoporos Int.-   20. Grant A M, Avenell A, Campbell M K, McDonald A M, MacLennan G S,    McPherson G C, Anderson FIH, Cooper C, Francis R M, Donaldson C,    Gillespie W J, Robinson C M, Torgerson D J, Wallace W A 2005 Oral    vitamin D3 and calcium for secondary prevention of low-trauma    fractures in elderly people (Randomised Evaluation of Calcium Or    vitamin D, RECORD): a randomised placebo-controlled trial. Lancet    365(9471):1621-8.-   21. Porthouse J, Cockayne S, King C, Saxon L, Steele E, Aspray T,    Baverstock M, Birks Y, Dumville J, Francis R, Iglesias C, Puffer S,    Sutcliffe A, Watt 1, Torgerson D J 2005 Randomised controlled trial    of calcium and supplementation with cholecalciferol (vitamin D3) for    prevention of fractures in primary care. BMJ 330(7498):1003.-   22. Aloia J F, Talwar S A, Pollack S, Yeh J 2005 A randomized    controlled trial of vitamin D3 supplementation in African American    women. Arch Intern Med 165(14):1618-23.-   23. Sanders K M, Stuart A L, Williamson E J, Simpson J A, Kotowicz    M A. Young D, Nicholson G C 2010 Annual high-dose oral vitamin D and    falls and fractures in older women: a randomized controlled trial.    JAMA 303(18):1815-22.-   24. Mendel C M 1989 The free hormone hypothesis: a physiologically    based mathematical model. Endocr Rev 10(3):232-74.-   25. Bikle D D, Siiteri P K, Ryzcn E, Haddad J G 1985 Serum protein    binding of 1,25-dihydroxyvitamin D: a reevaluation by direct    measurement of free metabolite levels. J Clin Endocrinol Metab    61(5):969-75.-   26. Safadi F F, Thornton P. Magiera H, Hollis B W, Gentile M, Haddad    J G, Liebhaber S A, Cooke N E 1999 Osteopathy and resistance to    vitamin D toxicity in mice null for vitamin D binding protein. J    Clin Invest 103(2):239-51.-   27. Chun R F, Lauridsen A L, Suon L, Zella L A, Pike J W, Modlin R    L, Martineau A R, Wilkinson R J, Adams J, Hewison M 2010 Vitamin    D-binding protein directs monocyte responses to 25-hydroxy- and    1,25-dihydroxyvitamin D. J Clin Endocrinol Metab 95(7):3368-76.-   28. Bikle D D, Gee E 1989 Free, and not total, 1,25-dihydroxyvitamin    D regulates 25-hydroxyvitamin D metabolism by keratinocytes.    Endocrinology 124(2):649-54.-   29. Bikle D D, Gee E, Halloran B, Kowalski M A, Ryzen E, Haddad J G    1986 Assessment of the free fraction of 25-hydroxyvitamin D in serum    and its regulation by albumin and the vitamin D-binding protein. J    Clin Endocrinol Metab 63(4):954-9.-   30. Shaham O, Wei R, Wang T J, Ricciardi C, Lewis G D, Vasan R S,    Carr S A, Thadhani R, Gerszten R E, Mootha V K 2008 Metabolic    profiling of the human response to a glucose challenge reveals    distinct axes of insulin sensitivity. Mol Syst Biol 4:214.-   31. Mazess R B, Barden IIS, Bisek J P, Hanson J 1990 Dual-energy    x-ray absorptiometry for total-body and regional bone-mineral and    soft-tissue composition. Am J Clin Nutr 51(6):1106-12.-   32. Marshall D, Johnell O, Wedel H 1996 Meta-analysis of how well    measures of bone mineral density predict occurrence of osteoporotic    fractures. BMJ 312(7041):1254-9.-   33. Hans D, Downs R W, Jr., Duboeuf F, Greenspan S, Jankowski L G,    Kiebzak G M, Petak S M 2006 Skeletal sites for osteoporosis    diagnosis: the 2005 ISCD Official Positions. J Clin Densitom    9(1):15-21.-   34. Vermeulen A, Verdonck L, Kaufman J M 1999 A critical evaluation    of simple methods for the estimation of free testosterone in serum.    J Clin Endocrinol Metab 84(10):3666-72.-   35. Liel Y, Edwards J, Shary J, Spicer K M, Gordon L, Bell Nil 1988    The effects of race and body habitus on bone mineral density of the    radius, hip, and spine in premenopausal women. J Clin Endocrinol    Metab 66(6):1247-50.-   36. Halioua L, Anderson J J 1989 Lifetime calcium intake and    physical activity habits: independent and combined effects on the    radial bone of healthy premenopausal Caucasian women. Am J Clin Nutr    49(3):534-41.-   37. Schneider G B, Benis K A, Flay N W, Ireland R A, Popoff S N 1995    Effects of vitamin D binding protein-macrophage activating factor    (DBP-MAF) infusion on bone resorption in two osteopetrotic    mutations. Bone 16(6):657-62.-   38. Fang Y, van Meurs J B, Arp P, van Leeuwen J P, Hlofman A, Pols H    A, Uitterlinden A G 2009 Vitamin D binding protein genotype and    osteoporosis. Calcif Tissue Int 85(2):85-93.-   39. Sinotte M, Diorio C, Berube S, Pollak M, Brisson J 2009 Genetic    polymorphisms of the vitamin D binding protein and plasma    concentrations of 25-hydroxyvitamin D in premenopausal women. Am J    Clin Nutr 89(2):634-40.-   40. Engelman C D, Fingerlin T E, Langefeld C D, Hicks P J, Rich S S,    Wagenknecht L E, Bowden D W, Norris J M 2008 Genetic and    environmental determinants of 25-hydroxyvitamin D and    1,25-dihydroxyvitamin D levels in Hispanic and African Americans. J    Clin Endocrinol Metab 93(9):3381-8.-   41. Al-oanzi Z H, Tuck S P, Raj N, Harrop J S, Summers G D, Cook D    B, Francis R M, Datta H K 2006 Assessment of vitamin D status in    male osteoporosis. Clin Chem 52(2):248-54.-   42. Lauridsen A L, Vestergaard P, HIermann A P, Brot C,    Ileickendorff L, Mosekilde L, Nexo E 2005 Plasma concentrations of    25-hydroxy-vitamin D and 1,25-dihydroxy-vitamin D are related to the    phenotype of Gc (vitamin D-binding protein): a cross-sectional study    on 595 early postmenopausal women. Calcif Tissue Int 77(1):15-22.-   43. Al-oanzi Z H, Tuck S P, Mastana S S, Summers G D, Cook D B,    Francis R M, Datta HK 2008 Vitamin D-binding protein gene    microsatellite polymorphism influences BMD and risk of fractures in    men. Osteoporos Int 19(7):951-60.-   44. Snijder M B, van Dam R M, Visser M, Deeg D J, Dekker J M, Bouter    L M, Seidell J C, Lips P 2005 Adiposity in relation to vitamin D    status and parathyroid hormone levels: a population-based study in    older men and women. J Clin Endocrinol Metab 90(7):4119-23.-   45. Cauley J A, Lui L Y, Ensrud K E, Zmuda J M, Stone K L, Hochberg    M C, Cummings S R 2005 Bone mineral density and the risk of incident    nonspinal fractures in black and white women. JAMA 293(17):2102-8.-   46. van Driel M, Koedam M, Buurman C J, Hewison M, Chiba H,    Uitterlinden A G, Pols H A, van Leeuwen J P 2006 Evidence for    auto/paracrine actions of vitamin D in bone: lalphahydroxylase    expression and activity in human bone cells. FASEB J 20(13):2417-9.-   47. Brent G A 1994 The molecular basis of thyroid hormone action. N    Engl J Med 331(13):847-53.-   48. van Hoof H J, Swinkels L M, Ross H A, Sweep C G, Benraad T J    1998 Determination of non-protein-bound plasma 1,25-dihydroxyvitamin    D by symmetric (rate) dialysis. Anal Biochem 258(2):176-83.-   49. Ginde A A, Mansbach J M, Camargo C A, Jr. 2009 Association    between serum 25-hydroxyvitamin D level and upper respiratory tract    infection in the Third National Health and Nutrition Examination    Survey. Arch Intern Med 169(4):384-90.-   50. Wactawski-Wende J, Kotchen J M, Anderson G L, Assaf A R, Brunner    R L, O'Sullivan M J, Margolis K L, Ockene J K, Phillips L, Pottern    L, Prentice R L, Robbins J, Rohan T E, Sarto G E, Sharma S,    Stefanick M L, Van Horn L, Wallace R B, Whitlock E, Bassford T,    Beresford S A, Black H R, Bonds D E, Brzyski R G, Caan B, Chlebowski    R T, Cochrane B, Garland C, Gass M. Hays J, Heiss G. Hendrix S L,    Howard B V, Hsia J. Hubbell F A, Jackson R D, Johnson K C, Judd H,    Kooperberg C L, Kuller LII, LaCroix A Z, Lane D S, Langer R D,    Lasser N L, Lewis C E, Limacher M C, Manson J E 2006 Calcium plus    vitamin D supplementation and the risk of colorectal cancer. N Engl    J Med 354(7):684-96.-   51. Wang T J, Pencina M J, Booth S L, Jacques P F, Ingelsson E,    Lanier K, Benjamin E J, D'Agostino R B, Wolf M, Vasan R S 2008    Vitamin D deficiency and risk of cardiovascular disease. Circulation    117(4):503-11.

Example 2 Bioavailable Vitamin D and Mineral Metabolism

Prior studies have yielded conflicting results regarding the associationbetween 25-hydroxyvitamin D (25(OH)D) levels and mineral metabolism inend-stage renal disease (ESRD). Described herein are experiments testingthe hypothesis that bioavailable vitamin D, the vitamin D fraction notbound to vitamin D binding protein (DBP), would associate more stronglywith measures of mineral metabolism than total levels. Eighty ninepatients with previously measured 25(OH)D and 1,25-dihydroxyvitamin D(1,25(OH)₂D) levels were identified from a cohort of incident U.S.dialysis patients. Stored serum samples were used to measure DBP,determine bioavailable 25(OH)D and 1,25(OH)₂D using previously validatedformulae, and examine associations with measures of mineral metabolismand demographic factors. Both bioavailable 25(OH)D and bioavailable1,25(OH)₂D were correlated with serum calcium (r=0.26, p=0.01 andr=0.23, p=0.02, respectively) whereas this association was absent forboth total 25(OH)D (r=0.01, p=0.92) and total 1,25(OH)₂D (r=0.08,p=0.44). Racial differences in DBP and total 25(OH)D, but notbioavailable vitamin D, were observed. In univariate and multivariateregression analysis, only bioavailable 25(OH)D was associated withparathyroid hormone levels (p=0.007 and p=0.02, respectively).Accordingly, bioavailable 25(OH)D levels are better correlated withmeasures of mineral metabolism than total 25(OH)D levels in patients onhemodialysis.

Chronic kidney disease-associated mineral and bone disorder (CKD-MBD) isone of the most appreciated metabolic complications of CKD. Asindividuals progress toward end-stage renal disease (ESRD), decliningrenal 1a-hydroxylase activity leads to decreased conversion of25-hydroxyvitamin D (25(OH)D) to the active 1,25-dihydroxyvitamin D(1,25(OH)₂D). These metabolic changes are believed to precipitate thehypocalcemia and secondary hyperparathyroidism that characterizeCKD-MBD. Although 1,25(OH)₂D is thought to be the biologically activemoiety, the majority of vitamin D circulates as 25(OH)D.(1) Low levelsof 25(OH)D are common in ESRD; 79% of patients initiating dialysis have25(OH)D levels below 30 ng/ml, and serum levels below this threshold arenearly universal among black ESRD patients.(2)

The free hormone hypothesis suggests that protein-bound hormones arerelatively inactive while those liberated from binding proteins are freeto exert biological activity.(3) For some hormones (e.g. testosterone),binding to albumin is considerably weaker than to a specific bindingprotein. Thus, albumin-bound hormone is often grouped with the freefraction and referred to as the “bioavailable” fraction. The majority(85-90%) of circulating 25(OH)D and 1,25(OH)D is tightly bound tovitamin D binding protein (DBP), with a smaller amount (10-15%) looselybound to albumin. Less than 1% of circulating vitamin D exists in afree, unbound form.(4,5) Described herein are experiments testing thehypothesis that the relationship between vitamin D and markers ofmineral metabolism (e.g. PTH and calcium) in ESRD would be strengthenedby use of DBP and albumin to determine bioavailable vitamin D levels.Given the patterns observed in other cohorts, it was furtherhypothesized that the lower 25(OH)D levels typically seen in blackdialysis patients would be associated with lower and not necessarilylower bioavailable vitamin D levels in this group.(6,7)

Results

Baseline characteristics of the 94 subjects included in this analysis,which are similar to those of a typical US hemodialysis population, aresummarized in Table 5. None of the included subjects were recorded asreceiving treatment with activated vitamin D, ergocalciferol, orcholecalciferol before initiating dialysis.

Mineral Metabolism and Vitamin D.

Baseline corrected calcium levels, measured within 14 days of chronichemodialysis initiation, were not associated with total levels of either25(OH)D (r=0.01, P=0.92) or 1,25(OH)₂D (r=0.08, P=0.44). In contrast,calcium levels correlated positively with both bioavailable 25(OH)D(r=0.26, p=0.01) and bioavailable 1,25(OH)₂D (r=0.23, p=0.02). Theserelationships are plotted in FIG. 2. A single individual with thehighest bioavailable 25(OH)D and bioavailable 1,25(OH)₂D appeared to bean outlier with respect to the observed relationships, with both levelsover 4 standard deviations above the mean. To examine the impact of thissingle data point, a sensitivity analysis was performed by repeating theanalysis with this individual excluded. The relationship of calcium withbioavailable 25(OH)D (r=0.30, p=0.003) and bioavailable 1,25(OH)₂D(r=0.27, p=0.008) were both somewhat strengthened.

Phosphorus levels demonstrated no association with either total levelsof 25(OH)D (r=0.14, P=0.19) or 1,25(OH)₂D (r=0.01, P=0.94). Similarly,neither bioavailable 25(OH)D (r=−0.10. P=0.32) nor bioavailable1,25(OH)₂D (r=−0.16, P=0.12) were significantly associated withphosphorus levels.

Alkaline phosphatase was not associated with either total orbioavailable forms of 25(OH)D or 1,25(OH)₂D (p>0.05 for allcomparisons).

The relationship between PTH and all four forms of vitamin D wereexamined in univariate and multivariate regression models. In univariatemodels, only bioavailable 25(OH)D was associated with PTH, with a −0.35log decrease in PTH for each log increase in bioavailable 25(OH)D(p=0.01). In a multivariate model controlling for age, gender, race, andsurvival status at one year, this relationship remained unchanged(β=−0.32, p=0.02). A third model adding calcium, phosphorus, andbioavailable 1,25(OH)2D levels demonstrated similar results (Table 7).As with the calcium findings, both the unadjusted and adjustedcoefficients became stronger when a single outlier was excluded(unadjusted: β=−0.40, p=0.003; adjusted: β=−0.36, p=0.01). In contrast,there was no significant association between total 25(OH)D and PTH (FIG.3).

Patient Factors and Vitamin D.

Older individuals had higher total 25(OH)D levels (r=0.31, P=0.003) andbioavailable 25(OH)D (r=0.21, p=0.04). Neither total nor bioavailable1,25(OH)₂D were associated with age. Female gender was associated withlower total 25(OH)D levels (median in men: 22.0 ng/dl, in women: 18.0ng/dl; p=0.03). While females had numerically lower median total1,25(OH)₂D and bioavailable 25(OH)D and 1,25(OH)₂D levels, none of thesedifferences were statistically significant.

Black individuals had lower total 25(OH)D levels (median: 15.2 vs 23.2ng/ml, p<0.001) but not bioavailable 25(OH)D levels (median: 3.8 vs. 2.8ng/ml, p=0.21). The contrast in racial differences between these twoforms of vitamin D was driven largely by lower DBP levels among blacks.This difference persisted even when examining only individuals whosurvived for one year on dialysis or those who died in this timeframe(Table 6). PTH levels did not differ significantly by race (median: 201pg/ml [black] vs. 168 pg/ml [white], p=0.47). Neither total norbioavailable 1,25(OH)₂D levels differed by race (p=0.07 and 0.49,respectively). Of note, we found no racial differences in systolic ordiastolic blood pressure, diabetes, or BMI.

The study was not specifically powered to address whether systolic anddiastolic blood pressure, BMI, and survival or a diagnosis of diabeticnephropathy or diabetes were associated with any form of vitamin D (datanot shown).

Sensitivity Analysis.

Sensitivity analyses were performed to address the possibility thaturemia might alter DBP's binding affinity with 25(OH)D or 1,25(OH)₂D.With DBP-binding coefficients that were 25% lower than those originallydetermined by Bikle, et al., (4,5) bioavailable measures of both 25(OH)D(r=0.26, p=0.01) and 1,25(OH)₂D (r=0.22, p=0.03) remained associatedwith corrected calcium. Similar results were observed with 25% highercoefficients (bioavailable 25(OH)D:r=0.27, p=0.009; bioavailable1,25(OH)₂D (r=0.24, p=0.02). Associations of bioavailable 25(OH)D withPTH remained statistically significant in both cases, with associationcoefficients changing less than 12% in either univariate or multivariateanalyses.

Discussion

Using a retrospective cohort of incident dialysis patients, therelationship between measures of mineral metabolism (including serumcalcium and PTHI) and both total and bioavailable levels of vitamin Dwas examined. Described herein are results indicating that bioavailable25(OH)D is associated with both corrected serum calcium levels and PTH,both of which are well-established measures of mineral metabolism inESRD, while total 25(OH)D demonstrates no such associations. This databuilds upon prior findings described elsewhere herein: analysis from twoseparate cohorts now support the hypothesis that bioavailable measuresof vitamin D, which take into account binding of vitamin D to albuminand DBP, are more relevant to biological outcomes than are total levels,which are currently the standard measure of vitamin D status.

Some in vitro studies suggest that DBP-binding limits vitamin D activityin multiple target cells.(8,9) Studies of DBP-null mice have shown thatthese animals display markedly reduced levels of 25(OH)D and 1,25(OH)₂Dcompared with wild-type mice, with a markedly reduced half-life.(10)Despite their low vitamin concentrations, when these mice are providedwith a steady source of dietary vitamin D, they show no differences inserum calcium, phosphorus, alkaline phosphatase, and PTH compared towild-type controls. These studies support the application of the freehormone hypothesis to vitamin D physiology, at least for some biologicalactions. Despite these findings, uptake of protein-bound hormone incells expressing megalin appears to be important for some processes, sothe biology underlying the findings described herein may be more complexthan is immediately apparent and warrants further investigation.(11,12)

Black patients were oversampled as the data presented in Example 1suggested blacks have lower DBP levels than whites, an observationsupported by the data described in this Example. As previously reported,the inventors and others observed that black race is associated withlower levels of total 25(OH)D.(2,13) As might be expected from these twoparallel racial differences (lower total 25(OH)D and lower DBP in blacksvs. whites), the levels of bioavailable 25(OH)D are similar, asdescribed herein.

Despite similar bioavailable D levels between racial groups, andassociation between bioavailable 25(OH)D and PTH, black patients hadnumerically higher PTH levels than their white counterparts. Though thisdifference was not statistically significant in the sample used herein,larger samples from this cohort have found significantly higher levelsof PTH in black individuals.(14) Bioavailable 25(OH)D did not differ byrace, yet were negatively associated with PTH, suggesting that racialdifferences in PTH are not primarily driven by differences in 25(OH)D.Indeed, others have found that PTH levels in blacks are higher thanthose in whites, even in states of 25(OH)D sufficiency.(13)

Several studies have attempted to assess the metabolic consequences oflow 25(OH)D levels in advanced CKD and ESRD, but have yieldedconflicting results. Ergocalciferol, a form of nutritional vitamin Dthat can increase 25(OH)D levels, appears to affect parathyroid hormone(PTH) levels in stage 3, but not in stage 4, CKD.(15,16) Moreover, somestudies have demonstrated a significant association between 25(OH)Dlevels and PTH in ESRD,(17-19) while others have not.(20,21)Associations between 25(OH)D and serum calcium have been similarlymixed.(2,17,22)

This contradictory data has led to confusion about the role thatrepeating 25(OH)D (e.g. with nutritional forms of vitamin D such ascholecalciferol or ergocalciferol) plays in the management of patientswith ERSD.(23,24) In order to study the role of vitamin D insufficiencyand identify patients who are most likely to benefit from repletion, itis critical to have a biologically relevant measure of vitamin D status.Notably, the experiments described herein failed to find any significantlink between total or bioavailable 1,25(OH)₂D and relevant measures ofmineral metabolism, echoing the general consensus that circulating serumlevels of the active hormone are not useful as a measure of vitamin Dstatus.(1)

A relationship between survival and vitamin D status was not found,though this sample had considerably less power to detect thisrelationship than prior studies, which have found that severe vitamin Dinsufficiency (typically defined levels<10 ng/ml) is associated withincreased mortality.(1925,26)

None of the individuals in this analysis, who initiated dialysis in 2004or 2005, had been treated with activated vitamin D analogs prior toinitiating dialysis.

PTH is commonly used as a proxy for metabolic bone disease in dialysispatients, but has an imperfect association with bone disease.(27) Bonebiopsies and non-invasive measures of bone density and structure werenot available in this study and are potential targets for futureanalyses. As described in Example 1 herein, a relationship betweenbioavailable 25(OH)D and bone density in a healthy population,(6) hasbeen demonstrated but it is not certain that this relationship willextend to the ESRD population given known alterations in mineralmetabolism. Metabolic changes that accompany ESRD and/or dialysis, aswell as genetic variants in DBP or other relevant proteins, have thepotential to influence binding of 25(OH)D to DBP. Whereas thesensitivity analysis did not indicate that these factors are likely toaffect the fundamental findings of this study, studies that directlymeasure bound and unbound fractions could improve upon the initialestimates and the equations used herein. Lastly, it is possible thatmeasured 25(OH)D levels in this study were influenced by levels of24,25(O)₂D. Confirmation of the findings described herein with assaysable to differentiate 25(OH)D, 24,25(OH)₂D, and 1,24,25(OH)₃D mayfurther elucidate these biological relationships.

This study provides additional evidence to support the notion thatbioavailable, rather than total, levels of vitamin D may be morerelevant measures of vitamin D status with respect to its actions onmineral metabolism. While mineral metabolism has been the traditionalfocus of vitamin D actions, recent data suggest that its actions may bemore widespread, with effects on the immune response,(28)hypertension.(29) and insulin sensitivity.(30), among others.

Methods

Accelerated Mortality on Renal Replacement (ArMORR) is a nationallyrepresentative prospective cohort study of incident chronic hemodialysispatients (n=10,044) who began renal replacement between Jul. 1, 2004 andJul. 30, 2005 at one of 1,056 dialysis centers in the U.S. operated byFresenius Medical Care, North America (FMC).(31) The ArMORR datasetcontains a broad range of demographic and clinical data includingco-existing medical conditions, laboratory results, as well as serum andplasma samples. Clinical data were collected prospectively, entereduniformly into a central database by practitioners at the point of care.All clinical data arriving at Fresenius undergo rigorous data qualityassurance and quality control (QA/QC) auditing. Blood samples collectedfor clinical care were shipped to and processed by a central laboratory(Spectra East, Rockland, N.J. USA). After processing for routineclinical testing, remnant samples were shipped on ice to the ArMORRInvestigators where the samples were aliquoted and stored in liquidnitrogen. This study was approved by the Institutional Review Board ofthe Massachusetts General Hospital, which waived the requirement forinformed consent, and conducted in accordance with its ethical standardsand the Declaration of Helsinki.

Study Population.

Between Jul. 1, 2004 and Jun. 30, 2005, 10,044 incident hemodialysispatients were prospectively enrolled into ArMORR. Subjects wereidentified who had 25(OH)D and 1,25(OH)₂D levels previously measured aspart of a case-control survival study.(19) Based on prior results in ahealthy population, we set a minimum sample size of 80 subjects. Toensure adequate power for racial comparisons, an approximately equalnumber of black (n=24) and white (n=23) patients were randomly selectedfrom the controls, and an equal number of race-matched cases. Thus, thetotal sample size was n=94. Baseline laboratory values were measuredfrom samples collected within 14 days of dialysis initiation.

Assays.

Total 25(OH)D and 1,25(OH)₂D were previously measured from thawedsamples in duplicate using a commercially available radioimmunoassay(DiaSorin Inc, Stillwater, Minn., USA). The interassay coefficients ofvariation (CVs) for 25(OH)D were <3% at levels<30 ng/ml and for1,25(OH)₂D were <6.5% at levels<32.5 pg/ml. Intact PTH (1-84) wasmeasured using the Nichols Advantage Biointact-PTH assay by thecentralized laboratory.

DBP was measured in duplicate in thawed serum samples by commercialenzyme linked immunosorbent assay (ELISA) (R&D Systems, Minneapolis,Minn., Catalog Number DVDBP0) according to the manufacturer'sinstructions. The assay was conducted after diluting serum samples 1 to2,(00)0 in Calibrator Diluent RD6-11 (R&D Systems Part Number 895489).Inter-assay CV was 8.5% at a concentration of 40 μg/ml. The assayrecovered between 93 and 110% of a 100-200 μg/mL dose of exogenous DBPadded to human serum samples containing between 25-200 μg/mL ofendogenous DBP. There were no differences in the recovery of exogenousDBP in black patients or obese patients. The manufacturer reports nosignificant cross-reactivity with human albumin or vitamin D3. DBPlevels were below the detection limit in 5 black patients who diedwithin the first year of dialysis. These individuals were assigned a DBPvalue equal to the lowest detectable level (12.3 μg/dl).

Calculation of Bioavailable Vitamin D.

Equilibrium dialysis and centrifugal ultrafiltration dialysis havepreviously been used by some investigators to indirectly measure freevitamin D levels, allowing estimation of the binding affinity constantsfor 25(OH)D and 1,25(OH)₂D with DBP and albumin.(4,5,32) In thesestudies, calculated levels of free 25(OH)D and levels measured bycentrifugal ultrafiltration were highly correlated (r=0.925).(5)Bioavailable and free vitamin D were calculated as described in Example1 herein.

Bioavailable 1,25(011)₂D levels were determined using the same approachusing affinity constants previously derived by centrifugalultrafiltration dialysis.(4) These affinity constants were previouslyvalidated in both healthy and cirrhotic individuals,(4,5) but have notbeen directly assessed in hemodialysis patients. Therefore a sensitivityanalysis of the main findings was performed using DBP bindingcoefficients for 25(OH)D and 1,25(OH)₂D that were 25% higher or 25%lower than previously measured values.

Statistical Analysis.

Prior to analysis, given the role of albumin as a binding protein forboth vitamin D and calcium, serum calcium levels were corrected foralbumin using the following equation: corrected calcium=totalcalcium+0.8*(4-albumin).(34) Spearman correlation analysis was performedto assess linear associations. Group comparisons of vitamin D levelswere performed using the Wilcoxon rank sum test. To examinemultivariable associations between bioavailable vitamin D and PTH, bothvariables (because of non-normal distribution) were natural-logtransformed and analyzed using multivariate linear regression. Allanalyses were conducted using STATA Statistical Software (CollegeStation, Tex.) version 11.

REFERENCES

-   1. Holick M F. Vitamin D deficiency. N Engl J Med. 2007 Jul.    19:357(3):266-281.-   2. Bhan I, Burnett-Bowie S-A M, Ye J, Tonelli M, Thadhani R.    Clinical measures identify vitamin D deficiency in dialysis. Clin J    Am Soc Nephrol. 2010 March; 5(3):460-467.-   3. Mendel C M. The free hormone hypothesis: a physiologically based    mathematical model. Endocr Rev. 1989 August; 10(3):232-274.-   4. Bikle D D, Siiteri P K, Ryzen E, Haddad J, Gee E. Serum Protein    Binding of 1,25-Dihydroxyvitamin D: A Reevaluation by Direct    Measurement of Free Metabolite Levels. J Clin Endocrinol Metab. 1985    Nov. 1; 61(5):969-975.-   5. Bikle D D, Gee E, Halloran B, Kowalski M A, Ryzen E, Haddad J G.    Assessment of the Free Fraction of 25-Hydroxyvitamin D in Serum and    Its Regulation by Albumin and the Vitamin D-Binding Protein. J Clin    Endocrinol Metab. 1986 Oct. 1; 63(4):954-959.-   6. Powe C E, Ricciardi C, Berg A H. Erdenesanaa D, Collerone G,    Ankers E, et al. Vitamin D-binding protein modifies the vitamin    D-bone mineral density relationship. J Bone Miner Res. 2011 July;    26(7):1609-1616.-   7. Powe C E, Seely E W, Rana S, Bhan I, Ecker J L, Karumanchi S A,    et al. First trimester vitamin D, vitamin D binding protein, and    subsequent preeclampsia. Hypertension. 2010 October; 56(4):758-763.-   8. Chun R F, Lauridsen A L, Suon L, Zella L A, Pike J W, Modlin R L,    et al. Vitamin D-binding protein directs monocyte responses to    25-hydroxy- and 1,25-dihydroxyvitamin D. J Clin Endocrinol Metab.    2010 July; 95(7):3368-3376.-   9. Bikle D D, Gee E. Free, and not total, 1,25-dihydroxyvitamin D    regulates 25-hydroxyvitamin D metabolism by keratinocytes.    Endocrinology. 1989 February; 124(2):649-654.-   10. Safadi F F, Thornton P, Magiera H, Hollis B W, Gentile M, Haddad    J G, et al. Osteopathy and resistance to vitamin D toxicity in mice    null for vitamin D binding protein. J Clin Invest. 1999 January;    103(2):239-251.-   11. Hammes A, Andreassen T K, Spoelgen R, Raila J. Hubner N, Schulz    H, et al. Role of endocytosis in cellular uptake of sex steroids.    Cell. 2005 Sep. 9; 122(5):751-762.-   12. Lundgren S, Carling T, Hjälm G, Juhlin C, Rastad J, Pihlgren U,    et al. Tissue distribution of human gp330/megalin, a putative    Ca(2+)-sensing protein. J. Histochem. Cytochem. 1997 March;    45(3):383-392.-   13. Gutiérrez O M, Farwell W R, Kermah D, Taylor E N. Racial    differences in the relationship between vitamin D, bone mineral    density, and parathyroid hormone in the National Health and    Nutrition Examination Survey. Osteoporosis Int. 2011 June;    22(6):1745-1753.-   14. Wolf M, Betancourt J, Chang Y, Shah A, Teng M, Tamez H, et al.    Impact of Activated Vitamin D and Race on Survival among    Hemodialysis Patients. J Am Soc Nephrol. 2008 Apr. 9;    19(7):1379-1388.-   15. Zisman A L, Hristova M, Ho L T, Sprague S M. Impact of    Ergocalciferol Treatment of Vitamin D Deficiency on Serum    Parathyroid Hormone Concentrations in Chronic Kidney Disease. Am J    Nephrol. 2007; 27(1):36-43.-   16. Al-Aly Z, Qazi R A, González E A, Zeringue A, Martin K J.    Changes in serum 25-hydroxyvitamin D and plasma intact PTH levels    following treatment with ergocalciferol in patients with CKD. Am J    Kidney Dis. 2007 Jul. 1; 50(1):59-68.-   17. Jean G, Charra B, Chazot C. Vitamin D Deficiency and Associated    Factors in Hemodialysis Patients. J Ren Nutr. 2008 Sep.;    18(5):395-399.-   18. Ravani P, Malberti F. Tripepi G, Pecchini P, Cutrupi S.    Pizini P. et al. Vitamin D levels and patient outcome in chronic    kidney disease. Kidney Int. 2009 Jan. 1; 75(1):88-95.-   19. Wolf M, Shah A, Gutierrez O, Ankers E, Monroy M, Tamez H, et al.    Vitamin D levels and early mortality among incident hemodialysis    patients. Kidney Int. 2007 Oct. 1; 72(8):1004-1013.-   20. González E A, Sachdeva A, Oliver D A, Martin K J. Vitamin D    insufficiency and deficiency in chronic kidney disease. A single    center observational study. Am J Nephrol. 2004 August;    24(5):503-510.-   21. London G M, Guèrin A P, Verbeke F H, Pannier B, Boutouyrie P,    Marchais S J, et al. Mineral metabolism and arterial functions in    end-stage renal disease; potential role of 25-hydroxyvitamin D    deficiency. J Am Soc Nephrol. 2007 Feb. 1; 8(2):613-620.-   22. London G M, Marty C, Marchais S J, Guerin A P, Metivier F, de    Vernejoul M-C. Arterial calcifications and bone histomorphometry in    end-stage renal disease. J Am Soc Nephrol. 2004 Jul. 1;    15(7):1943-1951.-   23. Nigwekar S U, Bhan I, Thadhani R. Nutritional vitamin D in    dialysis patients: what to D-iscem?Nephrology Dialysis    Transplantation. 2011 March; 26(3):764-766.-   24. Bhan I, Hewison M, Thadhani R. Dietary vitamin D intake in    advanced CKD/ESRD. Semin Dial. 2010 June; 23(4):407-410.-   25. Drechsler C. Pilz S, Obermayer-Pietsch B, Verduijn M, Tomaschitz    A, Krane V, et al. Vitamin D deficiency is associated with sudden    cardiac death, combined cardiovascular events, and mortality in    haemodialysis patients. Eur Heart J. 2010 September;    31(18):2253-2261.-   26. Drechsler C, Verduijn M, Pilz S, Dekker F W, Krediet R T, Ritz    E, et al. Vitamin D status and clinical outcomes in incident    dialysis patients: results from the NECOSAD study. Nephrol Dial    Transplant. 2011 March; 26(3): 1024-1032.-   27. Ott S M. Review article: Bone density in patients with chronic    kidney disease stages 4-5. Nephrology. 2009 June; 14(4):395-403.-   28. Bhan I, Camargo C A, Wenger J, Ricciardi C, Ye J, Borregaard N,    et al. Circulating levels of 25-hydroxyvitamin D and human    cathelicidin in healthy adults. J Allergy Clin Immunol. 2011 May;    127(5):1302-4.e1.-   29. Forman J P, Giovannucci E, Holmes M D, Bischoff-Ferrari H A,    Tworoger S S, Willett W C, et al. Plasma 25-hydroxyvitamin D levels    and risk of incident hypertension. Hypertension. 2007 May;    49(5):1063-1069.-   30. Chonchol M, Scragg R. 25-Hydroxyvitamin D, insulin resistance,    and kidney function in the Third National Health and Nutrition    Examination Survey. Kidney Int. 2007 Jan. 1; 71(2):134-139.-   31. Gombart A F, Bhan I, Borregaard N, Tamez H, Camargo C A,    Koeffler H P. et al. Low plasma level of cathelicidin antimicrobial    peptide (hCAP18) predicts increased infectious disease mortality in    patients undergoing hemodialysis. Clin. Infect. Dis. 2009 Feb. 15;    48(4):418-424.-   32. van Hoof H J, Swinkels L M, Ross H A, Sweep C G, Benraad T J.    Determination of non-protein-bound plasma 1,25-dihydroxyvitamin D by    symmetric (rate) dialysis. Anal. Biochem. 1998 May 1;    258(2):176-183.-   33. Vermeulen A, Verdonck L, Kaufman J M. A critical evaluation of    simple methods for the estimation of free testosterone in serum. J    Clin Endocrinol Metab. 1999 October; 84(10):3666-3672.-   34. Correcting the calcium. Br Med J. 1977 Mar. 5; 1(6061):598.

Example 3 Bioavailable Vitamin D in Pregnant Women

A pilot study in pregnant women enrolled in the MOMS cohort wasconducted to determine if VDBP modifies the relationship between firsttrimester 25(OH)D levels and subsequent development of preeclampsia orgestational diabetes. Although the sample sizes in pregnant women weresmall, total 25(OH)D levels were significantly lower among blacks (18.8[10.8-24.2] ng/mL, n=9), Hispanics (21.4 [16.7-27.7] ng/mL n=61), andAsians (22.5 [14.8-29.7] ng/ml, n=8) compared to white non-Hispanics(32.1 [26.3-36.6] ng/mL, n=127; P<0.03 for all comparisons). VDBP levelswere significantly lower in blacks (97.2 [73.6-361.2] μg/ml) compared towhites (504 [338-700]μg/ml, P<0.001). Bioavailable 25(OH)D levels weresimilar in white and black subjects.

Example 4 Bioavailable Vitamin D and Racial Variability

Black individuals consistently have low 25-hydroxy vitamin D(25[OH)D]levels¹⁵⁻¹⁹ and are at especially high risk for poor outcomeslinked to vitamin D sufficiency.¹⁵ Paradoxically, blacks have higher BMDand a lower risk of osteoporosis than whites.²⁰ Although 25(OH)D is themarker currently considered most suitable for assessing vitamin Dstatus,^(2,21) studies have shown a weaker relationship of 25(OH)D toBMD in blacks and other minorities compared to whites.^(18,19,22,23)Importantly, the BMD-vitamin D relationship in blacks needsclarification.¹⁸⁻²⁰ and questions about how to define clinicallyrelevant vitamin D insufficiency, the impact of race on measures ofvitamin D status, how to reliably identify who needs supplementationwith vitamin D (and with how much) to favorably impact disease outcomesremain unanswered.^(2,15)

The research described herein has the potential to redefine who isvitamin D deficient and to significantly improve diagnosis and theefficiency of strategies for prophylaxis and treatment. The hypothesisthat invokes the free hormone hypothesis in regards to bioavailablevitamin D can be investigated using a large cohort of black and whitesubjects enrolled in the Healthy Aging in Neighborhoods of Diversityacross the Life Span (HANDLS) study. Studies described in Examples 1, 2and 3 herein in various populations (healthy volunteers, individualswith end-stage renal disease (ESRD), pregnant women) measured VDBP,total 25(OH)D and calculated free 25(OH)D. These results indicate that25(OH)D levels are directly correlated with VDBP levels and inverselycorrelated with free 25(OH)D levels. The data also reveal substantialracial differences in VDBP levels. Blacks had 25-60% lower VDBP levelsthan whites, while free and bioavailable 25(OH)D levels were similar. Inhealthy adults, non-VDBP bound 25(OH)D levels were strongly associatedwith BMD, whereas total 25(OH)D levels and PTH were not.²³ Afteradjusting for race in patients with ESRD, both VDBP and free 25(OH)D(but not total 25(OH)D), were associated with PTH.³¹ These correlations(or lack thereof) between unbound vs total 25(OH)D and BMD and racialdifferences in VDBP levels led to the hyposthesis that VDBP modifies therelationship between BMD and 25(OH)D.²³ Total 25(OH)D may not faithfullyreflect physiologically relevant vitamin D status, particularly inrelation to BMD and questions the clinical relevance of 25(OH)D assaysin all races.³³

VDBP levels may be the previously unrecognized link that explains theBMD-vitamin D paradox in blacks. Using stored blood samples from blackand white subjects enrolled in HANDLS (n˜2,200), the followinghypotheses can be tested: 1. Blacks have lower levels of VDBP and total25(OH)D levels, but similar levels of free and bioavailable 25(OH)D aswhites. 2. Free and bioavailable 25(OH)D levels are independentlyassociated with BMD in blacks and whites, inversely and linearlyassociated with PTH levels, and these associations are stronger thanthose between total 25(OH)D and BMD.

Aim 1: To determine VDBP, total and bioavailable 25(OH)D, and PTH levelsin blacks vs whites.

Aim 2: To test for associations between BMD, bioavailable 25(OHi)D, andPTH compared to total 25(OH)D in blacks vs whites.

The studies described herein, (1) apply well-established mechanisms ofhormone biology; (2) demonstrate that bioavailable/free 25(OH)D is morestrongly associated with outcomes than total 25(OH)D; (3) can aid futureclinical trial designs in which vitamin D insufficiency will beredefined: (4) aim to individualize diagnosis and treatment based onrace to improve therapeutic and cost-efficiency of our limited healthcare resources, and (5) question current public health paradigms forclinical decision-making about who is vitamin D deficient, who should betreated, and who can avoid being treated.

25(OH)D Insufficiency is Even More Common in Blacks than in Whites.

According to current cutoffs defining vitamin D deficiency in terms of25(OH)D, approximately 50% of African Americans in the US are eitherchronically or seasonally at risk.⁴⁸ Racial disparities in vitamin Dstatus between blacks and whites may arise from insufficient dietaryintake or impaired conversion of vitamin D by sunlight due to skinpigmentation.¹⁶ Holick⁴⁸ reported that in Boston, 84% of black men andwomen>65 years old were vitamin D deficient at the end of the summer,which is typically when vitamin D levels are highest. Deficiency wasattributed to several factors including insufficient milk intake becauseof lactose intolerance, decreased synthesis of vitamin D3 in the skindue to pigmentation, and avoidance of sun exposure to minimize skinpigmentation.⁴⁸ According to the 1988-1994 National Health and NutritionExamination Survey (NHANES)III, 42% of black women (15-49 years old)were vitamin D deficient at the end of the winter compared to 4% ofwhite women.⁴⁹ Concentrations in serum of 25(OH)D measured duringdifferent seasons showed substantially lower levels in blacks (adjustedfor body weight and vitamin D intake) throughout all seasons and smallerseasonal increases during summer months than whites.⁵⁰ These racialdifferences are reinforced in more recent NHANES 2001-2006 data.^(22,51)Table 9 summarizes several representative studies reporting vitamin Dlevels in blacks vs whites. In most studies, the discrepancy betweenraces does not extend to 1,25 (OH)₂D levels, which are generally similarin blacks and whites.^(50,52)

As described in Example 2 herein, data from ArMORR (AcceleratedMortality in Renal Replacement which includes ESRD patients at theinitiation of dialysis prior to any vitamin D replacement) demonstratedthat mean 25(OH)D levels were 23.2±13.7 (SD) ng/mL in whites (n=653) and16.9±10.9 in blacks (n=372) (P<0.05). Similar results were found from arandomly-selected race-matched sample from ArMORR in which total 25(OH)Dwas 27.3±15.3 ng/mL in whites (n=23) and 16.4±10.1 in blacks(n=24)(P=0.004).³¹ Given that: (1) the incidence of vitamin Dinsufficiency is consistently higher in blacks than whites; (2) therange of total 25(OH)D cutoff levels used to define insufficiency iswide; and (3) correlation between 25(OH)D with mineral markers and BMDis lacking,^(23,40) the public health implications of continued relianceon total 25(OH)D for diagnosis and treatment are broad. The marker usedto diagnose and supplement vitamin D deficient states must beappropriate for racially diverse populations. Based on the datadescribed in Example 2, it is hypothesized that total 25(OH)D may not beuniformly applicable to all races. The definition of vitamin Dinsufficiency needs to be revisited.

Improving the understanding of vitamin D status among different races isexpected to have broadly significant therapeutic and public healthimplications. Specifically, by improving knowledge about vitamin Dbiology, the IOM's public health position on vitamin D replacement canbe refined to consider race and VDBP levels when determining vitamin Dtargets.

The inventors have previously reported a curvilinear relationshipbetween PTH and total 25(OH)D that PTH in older hospitalized patients.⁴⁰PTH was inversely correlated to total 25(OH)D levels<15 ng/mL. A similarstrong inverse correlation was observed in a large (n=825) cohort ofincident dialysis patients.⁴³ At higher total 25(OH)D levels (≧15ng/mL), the correlation with PTH was not as clear.⁴⁰ As described inExample 1, in younger healthy subjects whose mean total 25(OH)D levelswere 25.7±11.1 ng/mL (64.2=27.7 nmol/L), PTH was not correlated withtotal, bioavailable 25(OH)D, or BMD. This suggests that the associationbetween free or bioavailable 25(OH)D levels and BMD is not mediated viaPTH in individuals whose vitamin D status is relatively normal. However,due to the relatively small sample size of the healthy cohort, thesecorrelations may not have been evident. Example 2 indicates that, afteradjusting for race, free 25(OH)D (but not total 25[OH]D) correlates withPTH.

The large HANDLS dataset (n˜2,200) can be used to determine if PTH isbetter and more linearly correlated with free- or bioavailable 25(OH)Dthan with total 25(OH)D. Additionally, the question of whether racialdifferences exist among these variables can be explored. By followingsubjects with BMD measurements at baseline for changes over time in theHANDLS cohort, the measures of vitamin D which best predict changes inBMD in different racial groups can be identified.

Why do Blacks have Lower Levels of Vitamin D yet Higher BMD than OtherRaces?

Despite having lower levels of 25(OH)D, blacks have higher BMD^(19,20)and a lower risk of osteoporotic fractures than whites.^(20,58,59)Although factors other than vitamin D are likely to contribute, it hasbeen hypothesized that blacks have adaptive responses that protect theskeleton even when 25(OH)D is low. 16 Skeletal resistance to parathyroidhormone (PTH) activity and bone-sparing adaptations that promotebeneficial skeletal effects of active vitamin D (1,25 dihydroxyvitamin D[1,25(OH)₂D]) have been proposed to explain this paradox.¹⁶ For example,moderately low 25(OH)D can induce PTH-stimulated synthesis of 1,25(OH)₂Din the kidney.^(60,61) Using data from NHANES 2003-2006, Gutierrez andcolleagues²² observed that BMD decreased (p<0.01) with serum 25(OH)D andcalcium intake among whites and Mexican-Americans, but not among blacks(p=0.2).²² They proposed that relationships between 25(OH)D, BMD, andPTH differ by race. Other studies have also shown that the relationshipbetween 25(OH)D and BMD in blacks is weaker than in whites or isnonexistent.^(18,19)

Higher PTH levels have been reported in blacks than innon-blacks.^(22,31) Lower PTH may be related to low 25(OH)D, anadaptation to minimize urinary calcium losses and increase 1,25(OH)₂Dactivity. However, racial differences in PTH level persist even whentotal 25(OH)D is high,²² and PTH suppression by 25(OH)D may occur at alower threshold in blacks versus non-blacks.^(22,62) Thus, in additionto racial differences between BMD and total 25(OH)D levels, therelationships between PTH and free- and bioavailable 25(OH)D vs total25(OHi)D may also differ by race. It is proposed herein that the VDBPhypothesis will help explain or better understand these relationships.

Although the relationship of VDBP, 25(OH)D, and BMD has not been clearlyestablished in humans, animal studies suggest a role for VDBP inmodulating the rates of bioavailability, activation, and end-organresponsiveness of vitamin D metabolism.⁶⁶ as well as a role in theBMD-25(OH)D paradox in blacks. That is, despite having lower total25(OH)D levels, free and bioavailable 25(OH)D levels in blacks may infact be normal or even higher than normal, as a result of relatively lowVDBP concentrations as explained by the free-hormone hypothesis.Conversely, skin with light pigment captures vitamin D from theenvironment more readily, and higher VDBP levels may be an adaptationfor regulating bioavailable vitamin D. Genetic data are collected inHANDLS and phenotyping has been performed by the HANDLS investigatorswith results published in peer reviewed journals.^(67,68)

The significance of circulating VDBP levels with regard to thebiological activity of vitamin D in humans isunclear.^(2,18,19,21,22,24-26,30) Several properties that influence thebioavailability of vitamin D are analogous to those of the lipid-solubleandrogen hormone, testosterone (T). In the circulation, total T is 60%bound to sex hormone binding globulin (SHBG), whereas 38% isalbumin-bound and 2% is available as free-T.⁷⁵ Non-SHBG-bound T and freeT are the biologically active components of circulating T (bio-T).⁷⁶ Themethod used to calculate bio-T includes measured values of total T,albumin, SHBG and their binding constants into a mathematical model oftripartite binding. A strong correlation exists between measured bio-Tand calculated bio-T.⁷⁷ Using similar methods, concentrations of freeand bioavailable vitamin D may be calculated using measured affinityconstants to VDBP and albumin, as described in Example 1 and elsewhereherein. Calculated free D values are highly correlated with measuredvalues of free D, as validated by Bikle et al.^(38,70) and can be usedto estimate circulating free vitamin D concentrations.

Studies on the various circulating forms of vitamin D have shown thatvitamin D is 85-90% bound to VDBP, 10-15% is albumin-bound, and only 1%circulates freely.⁷⁰ Given that the majority of vitamin D is bound toVDBP, what is the role of VDBP in vitamin D physiology?Multiple animalmodels have demonstrated that VDBP is important as a high affinity serumreservoir for Vitamin D. VDBP-deficient animals have no high affinityserum carrier for the vitamin: as a consequence of this their serumvitamin concentrations are significantly decreased, and without a highaffinity carrier they rapidly excrete vitamin D in the urine. Togetherthese events cause mice to quickly develop a vitamin insufficiency whenput on diets low in Vitamin D.⁶⁶ Although these animals are prone torapidly develop vitamin insufficiency in the absence of dietaryvitamins, when dietary vitamin D is abundant, the animals are able tomaintain calcium homeostasis and do not appear to suffer fromhypovitaminosis.

In contrast, animals that express VDBP but are missing the receptorsrequired for renal resorption of VDBP from glomerular ultrafiltratedisplay an even more dramatic phenotype. A significant amount of VDBP(and albumin) are filtered by the glomerulus.^(72,78) The filtered VDBPand albumin are normally recovered in the proximal tubules, however, bymegalin/cubulin receptor-mediated endocytosis. In the absence ofmegalin, vitamin D is sequestered by VDBP to the urine, producing rapidand complete vitamin insufficiency even when provided with avitamin-enriched diet, and develop severe abnormalities in calciumhomeostasis and bone disease.⁷²

The conclusions drawn from these animal models support a model whereVDBP and its endocytic receptors act as serum reservoirs and provide amechanism for the prevention of urinary losses. Based upon tissueculture and animal model studies, it is unclear whether VDBP is involvedin the intracellular delivery of 25(OH)D to its target tissues. The highaffinity of extracellular VDBP for 25(OH)D may prevent spontaneousdissociation and diffusion into cells. Experiments in tissue culturehave shown that when vitamin D-responsive osteoblasts or monocytes aretreated with vitamin D, addition of VDBP in the media actually inhibits25(OH)D endocytosis and intracellular signaling.^(73,74) Furthermore,although VDBP-deficient mice have very low serum vitamin concentrations,if they are provided with sufficient vitamin D in their diet they do notsuffer from problems with calcium homeostasis, and they accumulatenormal amounts of 1,25(OH)₂D in their tissues.⁷³

Together, these biochemical, tissue culture, and animal model studiessuggest that although VDBP helps to prevent insensible urinary losses,retain serum vitamin levels, and maintain stable vitamin Dconcentrations between meals, it is not necessary for intracellulardelivery of 25(OH)D or its conversion to active 1,25(OH)₂D. If VDBP isnot absolutely necessary for intracellular delivery of 25(OH)D, andsince albumin-bound 25(OH)D is the second most abundant circulating formof the vitamin, an alternative pathway for vitamin D delivery to theproximal tubules of the kidney and other target tissues must beconsidered: endocytic delivery of albumin bound vitamin D. Albumin andVDBP are in the same protein family, and they both share megalin andcubulin as their endocytic receptors. The most important target forvitamin D delivery and its actions is the epithelium of the proximalconvoluted tubule (PCT). Recent evidence has unexpectedly emerged that alarge amount of albumin and VDBP are filtered through the glomerularbasement membrane and then resorbed in the proximal tubule.^(79,80) Inthe PCT, albumin is resorbed by cubulin-mediated endocytosis,⁷⁹ and VDBPis endocytosed by megalin binding (although cubilin may also play arole).^(72,78) Megalin and cubilin are part of the same receptorcomplex, and thus albumin and VDBP share very similar endocyticpathways. The fact that both these proteins (and any vitamin bound tothem) are delivered to the renal epithelium in such large amountssuggests that either one may be a vehicle of intracellular vitamindelivery. Once these proteins are inside, however, albumin may be theprincipal source of diffusible 25(OH)D given its low 25(OH)D affinitycompared to VDBP. It has recently been demonstrated that the largeamounts of endocytosed albumin are actually transported through therenal epithelium and back to the circulation intact by transcytosis.⁸¹Given the structural and evolutionary similarities between albumin andVDBP, and their shared endocytic receptors, it is hypothesized that VDBPalso participates in transcytosis.

The transcytosis of albumin (and possibly VDBP) through renal epithelialcells described above thus saves these cells from the burden of havingto degrade this mass in the lysosomes, which, based upon the estimatedamounts of albumin flux would be toxic to the cells. This recent findingis significant to vitamin D bioavailability in megalin-expressing targettissues because transcytosis would also provide for an ideal autocrinemechanism for efficient intracellular delivery of vitamin D that can beregulated. As albumin transits through the cells, it would release25(OH)D through spontaneous dissociation, providing a nearby source ofvitamin D to intracellular VDBPs, CYP27B1 hydroxylase, and VDRreceptors. In contrast, although VDBP should not release of much of itsbound vitamin during transit through the tubular epithelium, it wouldensure efficient recovery of 25(OH)D from the urine and its return tothe circulation.

Given this alternative model of vitamin D physiology, because themajority of total serum vitamin D is bound to VDBP, and becauseVDBP-bound vitamin represents an inert serum reservoir for vitaminstorage, although measurement of total 25(OH)D may indicate total bodystores, serum concentrations of total 25(OH)D will often not reflectvitamin bioactivity or sufficiency. Non-VDBP bound bioavailable vitaminD, on the other hand, may be a more faithful indicator of vitamin Dsufficiency. This model agrees with results from clinical studiesdescribed above herein and it provides a model that may explain thedifferences in 25(OH)D concentrations and differences in BMD betweenwhite and black subjects.

Comparing men with osteoporosis to men without osteoporosis, Al-oanziand colleagues²⁴ found that total 25(OH)D₃ levels were similar in bothgroups, but VDBP levels were significantly higher (P<0.001) Calculatedfree 25(OH)D3 and 1,25(OH)₂D3 were significantly lower in men with vswithout osteoporosis (p<0.00001).²⁴ Whereas total 25(OH)D3 levelsprovided only a crude estimate of vitamin D status, measurement offreehormones provided more biologically relevant information. Asdescribed herein in Example 1 (young healthy adults; n=49) BMD is notwell-correlated with total 25(OH)D but that free and bioavailable25(OH)D are much more strongly associated with BMD. Despite widevariation in VDBP concentrations in the study cohort, mean VDBP levelswere significantly lower in nonwhite than in white subjects (2.87±2.04,4.94±2.43, respectively; P<0.001). As demonstrated in Example 2 herein,a randomly selected group of racially-matched ESRD patients suggestedthat VDBP may also mediate vitamin D activity in ESRD. Lower VDBP levelswere found in blacks vs whites. Serum calcium correlated with free25(OH)D and 1,25(OH)₂D, but not with total 25[OH]D and free 25(OH)D andVDBP levels—but not total 25[OH]D—were significantly associated withPTH. As discussed in Example 3 herein, a study in pregnant women alsorevealed lower VDBP and total 25(OH)D levels in blacks vs whites,whereas bioavailable 25(OH)D was similar regardless of race.

On the basis of the data described herein, it is hypothesized that VDBPmodifies the relationship of BMD and 25(OH)D. More specifically,decreased concentrations of VDBP in nonwhites may explain the loweraverage concentrations of total 25(OH)D that have been consistentlyreported herein and elsewhere. Because of lower VDBP levels,bioavailable vitamin D will be normal or even increased, perhapsexplaining the apparent reduced risk of osteoporosis in blacks comparedto whites.

The invention described herein incorporates innovative hypotheses andapproaches that: 1. Apply mechanisms of hormone biology that arewell-established in other steroid hormone research (e.g. testosteroneand thyroid hormones) to explain the biological actions of vitamin D; 2.solves a complex problem that has puzzled clinicians for decades: 3. Islikely to impact the design of future clinical trials in which vitamin Dinsufficiency is redefined by decreased bioavailable vitamin D levels;4. Advances the applicability of the 25[OH]D assay that has long beenconsidered the gold-standard for determining vitamin D status andproposes a novel alternative (free/bioavailable D) that may solveperplexing inconsistencies in outcomes in different races that may besecondary to vitamin D status; 5, Advances a contemporary principle thatdiagnosis and treatment should be individualized based on, at least raceand gender, to improve the therapeutic and cost-efficiency of limitedhealth care resources.

Test Hypotheses in HANDLS Subjects.

The hypotheses described below can be explored (FIG. 5) by measuringtotal 25(OH)D and VDBP levels in stored serum samples from black andwhite subjects with baseline BMD measurements enrolled in HANDLS:33 1.Blacks have lower levels of VDBP and total 25(OH)D levels, but similarlevels of free and bioavailable 25(OH)D as whites. 2. Free andbioavailable 25(OH)D levels are independently associated with BMD inblacks and whites, and are inversely and linearly associated with PTHlevels, and the associations are stronger than those between total25(OH)D and BMD.

Levels of free and bioavailable vitamin D can be calculated from total25(OH)D. VDBP and serum albumin measurements as shown below.^(38,70,76)Data will be examined using multivariate analyses for the presence ofassociations between BMD and free- and bioavailable 25(OH)D, and forassociations between free- and bioavailable 25(OH)D and PTH in black vswhite subjects. Adjustments for covariates such as: history ofosteoporosis, age, sex, pre-menopausal status, smoking and alcohol use,oral vitamin D and calcium intake, use of bisphosphonates, exercise, andbody mass index can be made.

The HANDLS Study.

The HANDLS study is being conducted as part of the National Institutionof Aging Intramural Research Program (NIAIRP). Planned as a 20-yearlongitudinal study, HANDLS is designed to test hypotheses about agingand health disparities in minority and poor populations by evaluatingdifferences in rates and risks for pathological conditions associatedwith aging within diverse racial, ethnic, and economic groups. Data onphysical, genetic, biologic, demographic, psychosocial, andpsychophysiological parameters from a fixed cohort of 3,722 black andwhite participants in higher and lower socioeconomic status are beingcollected.

Population being Studied.

HANDLS participants are a fixed cohort of community-dwelling black andwhite adults aged 30-64. Participants in HANDLS have been recruited from12 pre-determined neighborhoods (groups of contiguous census tracts)comprising the geographic area of Baltimore city and South Baltimore.The population comprises a 4-way factorial cross of age (seven 5-yearage bands between 30-64), sex, race, and socioeconomic status indexed bypoverty status.

Variables Available.

The HANDLS database contains information obtained from householdinterviews of black and white participants about their health status,health service utilization, psychosocial factors, nutrition,neighborhood characteristics, and demographics. In addition, mobilemedical research vehicles deployed every three years collect data (over˜20 years) from the same participants on: medical history and physicalexamination, dietary recall, cognitive evaluation, psychophysiologyassessments including heart rate variability, arterial thickness,carotid ultrasonography, assessments of muscle strength and bonedensity, and laboratory measurements (blood chemistries, hematology,biomarkers of oxidative stress and biomaterials for genetic studies).

The serum levels of total 25(OH)D, VDBP, albumin, PTH can be examinedand bioavailable 25(OH)D calculated in all HANDLS subjects with baselineBMD measurements (˜2200). Associations between VDBP levels, total25(OH)D, and PTH levels can be tested for. In addition to baselineassessments, 5 follow-up triennial assessments are being collected aspart of HANDLS over ˜20 years. At least one follow-up measure at Year 3(and as many as possible for the duration of the funding period) for anexploratory evaluation of BMD and related outcomes (ie, osteoporosis,fractures).

Blood Available.

Approximately 2,200 participants of HANDLS have baseline BMDmeasurements and stored serum samples. Followup BMD data, and bloodsamples collected at Year 3 (and possibly Year 6 as funding allows) canbe obtained to evaluate associations between BMD and related outcomesand total, bioavailable, and free 25(OH)D over time.

Analysis Techniques.

Stored serum samples from the HANDLS repository collected at baselineand at Years 3 and 6 (as feasible) can be used. Total 25(OH)D levels canbe measured by high performance liquid chromatography/massspectrophotometry. Albumin can be measured on standard automatedplatforms. VDBP can be measured using a commercially available ELISA(R&D Systems) as described in Example 1 herein. Interassay CVs andintra-assay CVs for this assay are 5.7% and 7.4% respectively.

In order to validate the calculated estimates of free 25(OH)D and testwhether these methods are valid in both black and white subjects, free25(OH)D can be measured directly in a subset of samples (n=200) asdescribed previously.⁸² Since blood sampling dates are captured, seasoncan be included as a covariate in the modeling of vitamin Dmeasurements.

The PTH assay can be performed using an electrochemiluminescenceimmunoassay “ECLIA” on the Cobas E601 analyzer (Roche diagnostics). Theassay for determining intact PTH employs a sandwich test principle inwhich a biotinylated monoclonal antibody reacts with the N-terminalfragment (1-37) and a monoclonal antibody labeled with a rutheniumcomplex reacts with the C-terminal fragment (38-84). The test canperformed using 300 μL of EDTA plasma, which provides more stable testresults than serum.

Power Calculations.

To determine VDBP, total and bioavailable 25(OH)D, and PTH levels inblack vs white subjects. Power was calculated assuming a 2-sided t-testwith α=0.05 for differences in black vs white subjects for total 25(OH)D(primary variable of interest). Although no previous studies haveexplored this measure among different racial groups, bioavailable25(OH)D is derived from total 25(OH)D and is a secondary variable ofinterest and it can constitute a reliable proxy. The weighted means andstandard deviations were derived from 5 published reports^(18,19,83-85)constituting a total of 14,402 whites and 7,790 blacks with clinical anddemographic characteristics similar to what we expect from the HANDLSpopulation (relatively healthy population, predominantly middle-aged).From these studies, an expected mean and standard deviation were derivedfor total 25(OH)D among whites and blacks (28.07±3.71 ng/mL and17.88±12.28 ng/mL, respectively). The sample size of nearly 2,200 HANDLSsubjects can be expected to be adequate to detect primary and secondaryvariables of interest with a power of 0.8, even with the possibility ofinvalid measurements and missing data (estimates at <5% of availableobservations).

To Test for Associations Between BMD, Bioavailable 25(OH)D, and PTHCompared to Total 25(OH)D in Black Vs White Subjects.

To obtain estimates for whole body BMD, a random sample of 40 subjects,stratified by race, was selected from all available HANDLS subjects withnon-missing whole body BMD and total 25(OH)D measurements. Power wascalculated assuming a 2-sided p-value with α=0.05. Based on preliminaryanalysis, r=0.29 between total 25(OH)D and whole body BMD in whites andr=0.24 among blacks was obtained. Assuming a correlation betweenbioavailable 25(OH)D and whole body BMD to be ˜30% higher, based onfindings from the MACS study, 23 the linear relation between thesevariables can be approximated to be r=0.38 among whites and r=0.31 amongblacks. Based on these estimates, the reduced R² among whites and blacks(R²=0.14 and R2=0.10, respectively) for multiple linear regressionanalyses was calculated. The sample size of nearly 2,200 HANDLS subjectsis adequate to detect the primary and secondary variables of interestwith a power of 0.8, even with the possibility of invalid measurementsand missing data (estimates at <5% of available observations).

Statistical Analysis

Preliminary examination of VDBP, total, bioavailable 25(OH)D, and PTHlevels include assessment of the plausibility of values through observedranges. Measures of central tendency and dispersion can be documented.As initial examination of VDBP, total, bioavailable 25(OH)D, and PTHlevels have shown highly, right-skewed distributions, these variablescan be assessed for normality through use of visualization techniquessuch as quantile-quantile plots and testing by Shapiro-Wilkes tests bothfor the overall sample and stratified by race. If VDBP, total,bioavailable 25(OH)D, and PTH levels are found to be normallydistributed, mean levels between black and white subjects can becompared through the use of two-samples t-tests. Otherwise, thedistributions between black and white subjects will be compared throughthe use of nonparametric statistics such as Wilcoxon rank sum tests.Two-tailed p-values of <0.05 will be considered statisticallysignificant.

BMD can be preliminarily be examined for plausibility of values andmeasures of central tendency and dispersion will be documented. Thedistribution of BMD can be inspected both through visualization andthrough formal statistical testing. Exploratory data analysis can beperformed through the use of scatterplots to inspect the relationshipbetween BMD, total 25(OH)D, bioavailable 25(OH)D, and PTH. If a linearrelationship is found between variables, Pearson Product-MomentCorrelation Coefficient or Spearman's Rank Correlation Coefficient (inthe case of outliers) can be used to examine associations between BMD,total 25(OH)D, bioavailable 25(OH)D, and PTH. Based on the strength ofthe relationships found between bioavailable 25(OH)D, total 25(OH)D, andPTH and BMD, multivariable linear regression models can be constructedto determine the independent association of bioavailable 25(OH)D, total25(OH)D, and PTH to BMD adjusting for other covariates, includinghistory of osteoporosis, age, sex, pre-menopausal status, smoking andalcohol use, oral vitamin D and calcium intake, use of bisphosphonates,exercise, and body mass index. Seasonality can be explored in all modelsby controlling for month of data collection and if seasonality is found,a cyclic regression curve can be included in the models.

The relationships between covariates and BMD, vitamin D levels, and PTHcan be explored through the use of independent samples t-tests (forbinary covariates) and simple linear regression (for continuouscovariates) as well as through visualization techniques includingboxplots and scatterplot matrices. Predictive variables can be selectedthrough the use of Least Angle Regression (LAR)⁸⁶ or Least AbsoluteShrinkage and Selection Operator (LASSO)⁸⁷ since both have proved to bemore reliable than the traditional stepwise variable selection method.Covariates central to our hypotheses or that have been shown in previousliterature to be clinically relevant to 25(OH)D, total 25(OH)D, and PTHand BMD can be left in the models irrespective of selection. Modelassessment can be performed through a variety of procedures.Collinearity can be assessed through correlations between covariates andthe variance inflation factor (VIF). Residuals can be examined throughstandardized residual plots and normality can be tested by Shapiro-Wilktests. Component plus residual plots will provide a means to investigatethe linearity between continuous predictors with BMD controlling for theeffects of other predictors in the model. The degree of overlap betweenLOESS curves and the regression line can be used to assess linearity ofthe predictors. Diagnostics of influential data points can be identifiedthrough Cook's distances (D) where observations with (Di>4/n) will besuspected of influence. A jack-knife approach can be used to comparemodels with and without influential observations and the percentage oferror attributed to individual observations can be documented.Observations with high Cook's distances but biologically plausiblevalues can be considered for inclusion in the final models. The finalcandidates of models can be compared for Akaike Information Criterion(AIC) value as well as adjusted Rsquared value.

Inclusion and formal testing of interactions between all covariates,with special attention paid to the interaction between race andsocioeconomic status, can be incorporated into un-stratifiedmultivariable models. Nonsignificant interaction terms can be droppedfrom final models. Significant interaction terms can be visuallyinspected through the use of Trellis graphs and the associations betweencovariates and BMD can be interpreted according to the level of theinteractive predictor. Multivariable models cam be conducted stratifiedby race and will be analyzed in a similar manner with the exclusion ofinteraction terms including race.

As CVD has been linked to 25(OH)D status, exploratory analyses can beperformed using available data to detect any relationships betweenbioavailable, total 25(OH)D, and PTH and carotid intimal medialthickness and blood pressure at baseline.

REFERENCES For Example 4 and the Background of Invention Sections

-   1. Cauley J A, Lacroix A Z, Wu L, et al. Serum 25-hydroxyvitamin D    concentrations and risk for hip fractures. Ann Intern Med 2008;    149:242-50.-   2. Judd S E, Tangpricha V. Vitamin D deficiency and risk for    cardiovascular disease. Am J Med Sci 2009; 338:40-4.-   3. Martins D, Wolf M, Pan D, et al. Prevalence of cardiovascular    risk factors and the serum levels of 25-hydroxyvitamin D in the    United States: data from the Third National Health and Nutrition    Examination Survey. Arch Intern Med 2007; 167:1159-65.-   4. Wang T J, Pencina M J, Booth S L, et al. Vitamin D deficiency and    risk of cardiovascular disease. Circulation 2008; 117:503-11.-   5. Zittermann A, Schleithoff S S, Tenderich G, Berthold H K, Korfer    R, Stehle P. Low vitamin D status: a contributing factor in the    pathogenesis of congestive heart failure?J Am Coll Cardiol 2003;    41:105-12.-   6. Ooms M E, Roos J C, Bezemer P D, van der Vijgh W J, Bouter L M,    Lips P. Prevention of bone loss by vitamin D supplementation in    elderly women: a randomized double-blind trial. J Clin Endocrinol    Metab 1995; 80:1052-8.-   7. Dawson-Hughes B. Harris S S, Krall E A, Dallal G E. Effect of    calcium and vitamin D supplementation on bone density in men and    women 65 years of age or older. N Engl J Med 1997; 337:670-6.-   8. Dawson-Hughes B, Harris S S, Krall E A, Dallal G E, Falconer G,    Green C L. Rates of bone loss in postmenopausal women randomly    assigned to one of two dosages of vitamin D. Am J Clin Nutr 1995;    61:1140-5.-   9. Jackson R D., LaCroix A Z, Gass M, et al. Calcium plus vitamin D    supplementation and the risk of fractures. N Engl J Med 2006;    354:669-83.-   10. Bischoff-Ferrari H A, Willett W C, Wong J B, Giovannucci E.    Dietrich T, Dawson-Hughes B. Fracture prevention with vitamin D    supplementation: a meta-analysis of randomized controlled trials.    JAMA 2005; 293:2257-64.-   11. Bischoff-Ferrari H A, Willett W C, Wong J B, et al. Prevention    of nonvertebral fractures with oral vitamin D and dose dependency: a    meta-analysis of randomized controlled trials. Arch Intern Med 2009;    169:551-61.-   12. Chapuy M C, Arlot M E, Duboeuf F, et al. Vitamin D3 and calcium    to prevent hip fractures in the elderly women. N Engl J Med 1992;    327:1637-42.-   13. Wang L, Manson J E, Song Y, Sesso lID. Systematic review:    Vitamin D and calcium supplementation in prevention of    cardiovascular events. Ann Intern Med 2010; 152:315-23.-   14. Autier P, Gandini S. Vitamin D supplementation and total    mortality: a meta-analysis of randomized controlled trials. Arch    Intern Med 2007; 167:1730-7.-   15. Fiscella K, Franks P. Vitamin D, race, and cardiovascular    mortality: findings from a national U S sample. Ann Fam Med 2010;    8:11-8.-   16. Harris S S. Vitamin D and African Americans. J Nutr 2006;    136:1126-9.-   17. Orwoll E, Nielson C M, Marshall L M, et al. Vitamin D deficiency    in older men. J Clin Endocrinol Metab 2009; 94:1214-22.-   18. Bischoff-Ferrari H A, Dietrich T, Orav E J, Dawson-Hughes B.    Positive association between 25-hydroxyvitamin D levels and bone    mineral density: a population-based study of younger and older    adults. Am J Med 2004; 116:634-9.-   19. Ilannan M T, Litman IIJ, Araujo A B, et al. Serum    25-hydroxyvitamin D and bone mineral density in a racially and    ethnically diverse group of men. J Clin Endocrinol Metab 2008;    93:40-6.-   20. Cauley J A, Lui L Y, Ensrud K E, et al. Bone mineral density and    the risk of incident nonspinal fractures in black and white women.    JAMA 2005; 293:2102-8.-   21. Thacher T D, Clarke B L. Vitamin D insufficiency. Mayo Clin Proc    2011; 86:50-60.-   22. Gutierrez O M, Farwell W R, Kermah D, Taylor E N. Racial    differences in the relationship between vitamin D, bone mineral    density, and parathyroid hormone in the National Health and    Nutrition Examination Survey. Osteoporos Int 2010; E-Pub ahead of    print.-   23. Powe C E. Ricciardi C, Berg A H, et al. Vitamin D-binding    protein modifies the vitamin D-bone mineral density relationship. J    Bone Miner Res 2011; 26:1609-16.-   24. Al-oanzi Z H, Tuck S P, Raj N, et al. Assessment of vitamin D    status in male osteoporosis. Clin Chem 2006; 52:248-54.-   25. Lauridsen A L, Vestergaard P, Hermann A P, et al. Plasma    concentrations of 25-hydroxy-vitamin D and 1,25-dihydroxy-vitamin D    are related to the phenotype of Gc (vitamin D-binding protein): a    cross-sectional study on 595 early postmenopausal women. Calcif    Tissue Int 2005; 77:15-22.-   26. Sinotte M, Diorio C, Berube S, Pollak M, Brisson J. Genetic    polymorphisms of the vitamin D binding protein and plasma    concentrations of 25-hydroxyvitamin D in premenopausal women. Am J    Clin Nutr 2009; 89:634-40.-   27. Constans J, Hazout S, Garruto R M, Gajdusek D C, Spees E K.    Population distribution of the human vitamin D binding protein:    anthropological considerations. Am J Phys Anthropol 1985; 68:107-22.-   28. Fang Y, van Meurs J B, Arp P, et al. Vitamin D binding protein    genotype and osteoporosis. Calcif Tissue Int 2009; 85:85-93.-   29. Papiha S S, Allcroft L C, Kanan R M, Francis R M, Datta H K.    Vitamin D binding protein gene in male osteoporosis: association of    plasma DBP and bone mineral density with (TAAA)(n)-Alu polymorphism    in DBP. Calcif Tissue Int 1999; 65:262-6.-   30. Wang T J, Zhang F, Richards J B, et al. Common genetic    determinants of vitamin D insufficiency: a genome-wide association    study. Lancet 2010; 376:180-8.-   31. Powe C E K S, Pham J L, Ye J, Ankers E D, Ricciardi C E,    Thadhani R, Bhan I. Vitamin D Binding Protein Modifies Relationship    between Vitamin D and Calcium in Dialysis. In: American Society of    Nephrology Annual Meeting; Denver, Colo.; November 2010.-   32. Powe C E. Vitamin D Binding Protein as a Modifier of Vitamin D    Activity in Humans [MD Honors]. Cambridge: Harvard; 2011.-   33. Institute of Medicine. Dietary Reference Intakes for Calcium and    Vitamin D. In: http://www.iom.edu/˜/media/Files/Report %20Files/2010    Dietary-Reference-Intakes-for-Calcium-and-Vitamin-D/Vitamin %20D    %20and %20Calcium%202010%20Report %20Brief.pdf, ed.; November    2010:1-4.-   34. Holick M F. Vitamin D deficiency. N Engl J Med 2007:357:266-81.-   35. O'Riordan J L. Rickets in the 17th Century. Journal of Bone and    Mineral Research 2006; 21:1506-10.-   36. Gloth F M, 3rd, Gundberg C M, Hollis B W, Haddad J G, Jr., Tobin    J D. Vitamin D deficiency in homebound elderly persons. Jama 1995;    274:1683-6.-   37. Compher C W, Badellino K O, Boullata J I. Vitamin D and the    bariatric surgical patient: a review. Obes Surg 2008; 18:220-4.-   38. Bikle D D, Siiteri P K, Ryzen E, Haddad J G. Serum protein    binding of 1,25-dihydroxyvitamin D: a reevaluation by direct    measurement of free metabolite levels. J Clin Endocrinol Metab 1985;    61:969-75.-   39. Matsuoka L Y, Ide L, Wortsman J, MacLaughlin J A, Holick M F.    Sunscreens suppress cutaneous vitamin D3 synthesis. J Clin    Endocrinol Metab 1987; 64:1165-8.-   40. Thomas M K, Lloyd-Jones D M, Thadhani R I, et al.    Hypovitaminosis D in medical inpatients. N Engl J Med 1998;    338:777-83.-   41. Hamilton S A, McNeil R, Hollis B W, et al. Profound Vitamin D    Deficiency in a Diverse Group of Women during Pregnancy Living in a    Sun-Rich Environment at Latitude 32 degrees N. Int J Endocrinol    2010; 2010:917428.-   42. Tangpricha V, Pearce E N, Chen T C, Holick M F. Vitamin D    insufficiency among free-living healthy young adults. Amn J Med    2002; 112:659-62.-   43. Wolf M, Shah A, Gutierrez O, et al. Vitamin D levels and early    mortality among incident hemodialysis patients. Kidney Int 2007;    72:1004-13.-   44. Merewood A, Mehta S D, Grossman X, et al. Widespread vitamin D    deficiency in urban Massachusetts newborns and their mothers.    Pediatrics 2010; 125:640-7.-   45. Wortsman J, Matsuoka L Y, Chen T C, Lu Z, Hlolick M F. Decreased    bioavailability of vitamin D in obesity. Am J Clin Nutr 2000;    72:690-3.-   46. Holick M F. MrOs is D-ficient. J Clin Endocrinol Metab 2009;    94:1092-3.-   47. Vieth R, Bischoff-Ferrari H, Boucher B J, et al. The urgent need    to recommend an intake of vitamin D that is effective. Am J Clin    Nutr 2007; 85:649-50.-   48. Holick M F. Sunlight and vitamin D for bone health and    prevention of autoimmune diseases, cancers, and cardiovascular    disease. Am J Clin Nutr 2004; 80:1678S-88S.-   49. Nesby-O'Dell S, Scanlon K S, Cogswell M E, et al.    Hypovitaminosis D prevalence and determinants among African American    and white women of reproductive age: third National Health and    Nutrition Examination Survey. 1988-1994. Am J Clin Nutr 2002;    76:187-92.-   50. Harris S S, Dawson-Hughes B. Seasonal changes in plasma    25-hydroxyvitamin D concentrations of young American black and white    women. Am J Clin Nutr 1998; 67:1232-6.-   51. Diaz V A, Mainous A G, 3rd, Carek P J, Wessell A M, Everett C J.    The association of vitamin D deficiency and insufficiency with    diabetic nephropathy: implications for health disparities. J Am    Board Fam Med 2009; 22:521-7.-   52. Matsuoka L Y, Wortsman J, Chen T C, Holick M F. Compensation for    the interracial variance in the cutaneous synthesis of vitamin D. J    Lab Clin Med 1995; 126:452-7.-   53. Wolf M, Betancourt J, Chang Y, et al. Impact of activated    vitamin D and race on survival among hemodialysis patients. J Am Soc    Nephrol 2008; 19:1379-88.-   54. Bischoff-Ferrari H A, Giovannucci E, Willett W C, Dietrich T,    Dawson-Hughes B. Estimation of optimal serum concentrations of    25-hydroxyvitamin D for multiple health outcomes. Am J Clin Nutr    2006; 84:18-28.-   55. Parfitt A M. Osteomalacia and related disorders In: Avioli L V,    Krane, S. M., ed. Metabolic bone disease. 3rd ed. San Diego, Calif.:    Academic Press; 1998:327-86.-   56. Sahota O. Osteoporosis and the role of vitamin D and    calcium-vitamin D deficiency, vitamin D insufficiency and vitamin D    sufficiency. Age Ageing 2000; 29:301-4.-   57. Morris H A, Anderson P H. Autocrine and paracrine actions of    vitamin D. Clin Biochem Rev 2010; 31:129-38.-   58. Karagas M R, Lu-Yao G L, Barrett J A, Beach M L, Baron J A.    Heterogeneity of hip fracture: age, race, sex, and geographic    patterns of femoral neck and trochanteric fractures among the U S    elderly. Am J Epidemiol 1996; 143:677-82.-   59. Barrett J A, Baron J A, Karagas M R, Beach M L. Fracture risk in    the U.S. Medicare population. J Clin Epidemiol 1999; 52:243-9.-   60. Breslau N A. Normal and abnormal regulation of 1,25-(OH)2D    synthesis. Am J Med Sci 1988; 296:417-25.-   61. Need A G, Horowitz M, Morris H A, Nordin B C. Vitamin D status:    effects on parathyroid hormone and 1,25-dihydroxyvitamin D in    postmenopausal women. Am J Clin Nutr 2000; 71:1577-81.-   62. Aloia J F, Chen D G, Chen H. The 25(OH)D/PTH threshold in black    women. J Clin Endocrinol Metab 2010; 95:5069-73.-   63. Braun A. Bichlmaier R, Cleve H. Molecular analysis of the gene    for the human vitamin-D-binding protein (group-specific component):    allelic differences of the common genetic GC types. Hum Genet 1992;    89:401-6.-   64. Al-oanzi Z H, Tuck S P, Mastana S S, et al. Vitamin D-binding    protein gene microsatellite polymorphism influences BMD and risk of    fractures in men. Osteoporos Int 2008; 19:951-60.-   65. Anderson L N, Cotterchio M, Cole D E, Knight J A. Vitamin    D-Related Genetic Variants, Interactions with Vitamin D Exposure and    Breast Cancer Risk among Caucasian Women in Ontario. Cancer    Epidemiol Biomarkers Prev 2011.-   66. Safadi F F, Thornton P, Magiera H, et al. Osteopathy and    resistance to vitamin D toxicity in mice null for vitamin D binding    protein. J Clin Invest 1999; 103:239-51.-   67. Noren Hooten N, Abdelmohsen K, Gorospe M, Ejiogu N, Zonderman A    B, Evans M K. microRNA expression patterns reveal differential    expression of target genes with age. PLoS One 2010; 5:e 10724.-   68. Smith J G, Magnani J W, Palmer C, et al. Genome-Wide Association    Studies of the PR Interval in African Americans. PLoS Genet 2011;    7:EPub: e1001304.-   69. Mendel C M. The free hormone hypothesis: a physiologically based    mathematical model. Endocr Rev 1989; 10:232-74.-   70. Bikle D D, Gee E, Halloran B, Kowalski M A, Ryzen E, Haddad J G.    Assessment of the free fraction of 25-hydroxyvitamin D in serum and    its regulation by albumin and the vitamin D-binding protein. J Clin    Endocrinol Metab 1986; 63:954-9.-   71. Arnaud J, Constans J. Affinity differences for vitamin D    metabolites associated with the genetic isoforms of the human serum    carrier protein (DBP). Hum Genet 1993; 92:183-8.-   72. Leheste J R, Melsen F, Wellner M, et al. Hypocalcemia and    osteopathy in mice with kidney-specific megalin gene defect. FASEB J    2003; 17:247-9.-   73. Zella L A, Shevde N K, Hollis B W, Cooke N E, Pike J W. Vitamin    D-binding protein influences total circulating levels of    1,25-dihydroxyvitamin D3 but does not directly modulate the    bioactive levels of the hormone in vivo. Endocrinology 2008;    149:3656-67.-   74. Chun R F, Lauridsen A L, Suon L, et al. Vitamin D-binding    protein directs monocyte responses to 25-hydroxy- and    1,25-dihydroxyvitamin D. J Clin Endocrinol Metab 2010; 95:3368-76.-   75. Braunstein G D. In: Basic & Clinical Endocrinology. 5th ed.    Stamford, Conn.: Appleton & Lange; 1997; 422-452.-   76. Vermeulen A, Verdonck L, Kaufman J M. A critical evaluation of    simple methods for the estimation of free testosterone in serum. J    Clin Endocrinol Metab 1999; 84:3666-72.-   77. Emadi-Konjin P, Bain J, Bromberg I L. Evaluation of an algorithm    for calculation of serum “bioavailable” testosterone (BAT). Clin    Biochem 2003; 36:591-6.-   78. Nykjaer A, Fyfe J C, Kozyraki R, et al. Cubilin dysfunction    causes abnormal metabolism of the steroid hormone 25(OH) vitamin    D(3). Proc Natl Acad Sci USA 2001; 98:13895-900.-   79. Amsellem S, Gburek J, Hamard G, et al. Cubilin is essential for    albumin reabsorption in the renal proximal tubule. J Am Soc Nephrol    2010; 21:1859-67.-   80. Russo L M, Sandoval R M, McKee M, et al. The normal kidney    filters nephrotic levels of albumin retrieved by proximal tubule    cells: retrieval is disrupted in nephrotic states. Kidney Int 2007;    71:504-13.-   81. Russo D, Corrao S, Miranda I, et al. Progression of coronary    artery calcification in predialysis patients. Am J Nephrol 2007;    27:152-8.-   82. van Hoof H J, Swinkels L M, Ross H A, Sweep C G, Benraad T J.    Determination of non-protein-bound plasma 1,25-dihydroxyvitamin D by    symmetric (rate) dialysis. Anal Biochem 1998; 258:176-83.-   83. Gutierrez O M, Farwell W R, Kermah D, Taylor E N. Racial    differences in the relationship between vitamin D, bone mineral    density, and parathyroid hormone in the National Health and    Nutrition Examination Survey. Osteoporos Int 2011; 22:1745-53.-   84. Reis J P, Michos E D, von Muhlen D, Miller E R, 3rd. Differences    in vitamin D status as a possible contributor to the racial    disparity in peripheral arterial disease. Am J Clin Nutr 2008;    88:1469-77.-   85. Cauley J A, Danielson M E, Boudreau R, et al. Serum 25    hydroxyvitamin (OH)D and clinical fracture risk in a multiethnic    Cohort of women: The Women's health initiative (WHI). J Bone Miner    Res 2011.-   86. Efron B, IHastie T, Johnstone I, Tibshirani R. Least angle    regression. Ann Stat 2004; 32:407-99.-   87. Tibrishani R. Regression shrinkage and section via the lasso. J    Royal Stat Soc 1996:58:267-88.-   88. Wolf M, Sandler L, Munoz K, Hsu K, Ecker J L, Thadhani R. First    trimester insulin resistance and subsequent preeclampsia: a    prospective study. J Clin Endocrinol Metab 2002; 87:1563-8.

TABLE 1 Characteristics of the Study Population (n = 49). Data arepresented as n (%) for categorical variables and mean ± standarddeviation for continuous variables. Mean ± SD n (%) Age (years) 23.5 ±3.4  Body Mass Index (kg/m²) 22.43 ± 2.96  Sex Male 27 (55.1%) Female 22(44.9%) Race White 31 (63.3%) Non-White 18 (36.7%) Exercise Amount >120minutes per week 21 (42.9%) ≦120 minutes per week 26 (53.1%) Unknown 2(4.1%) Vitamin D Binding Protein (umol/L) 4.19 ± 2.49 Albumin (g/L)42.47 ± 3.94  Serum Calcium (mmol/L) 2.30 ± 0.19 Parathyroid Hormone(ng/L) 29.86 ± 8.25  Dietary Calcium Intake (mg/day) 925.85 ± 421.76Lumbar Spine Bone Mineral Density (g/cm²) 1.05 ± 0.14

TABLE 2 Serum Levels of Vitamin D. Total 25(OH)D levels were measuredalong with albumin and vitamin D binding protein (DBP). DBP-bound,albumin-bound, free, and bioavailable 25(OH)D (free and albumin bound)were calculated Mean ± SD Total 25(OH)D (nmol/L) 64.23 ± 27.70 DBP-Bound25(OH)D (nmol/L) 54.66 ± 26.32 Albumin-Bound 25(OH)D (nmol/L) 9.55 ±6.72 Free 25(OH)D (pmol/L) 25.37 ± 18.52 Bioavailable 25(OH)D (nmol/L)9.58 ± 6.74

TABLE 3 Bone Mineral Density, DBP Levels, and 25(OH)D Levels in SelectSubgroups. Values are reported as mean ± standard deviation. DBP =vitamin D binding protein. BMD = bone mineral density. OCP = oralcontraceptive pill. Groups were compared using t-tests after natural logtransformation of total 25(OH)D, DBP, bioavailable 25(OH)D levels andBMD. Total 25(OH)D Total DBP Bioavailable 25(OH)D L-Spine BMD n (nmol/L)(umol/L) (nmol/L) (g/cm²) Sex Male 27 52.79 ± 19.31 3.90 ± 2.09 8.36 ±5.36 1.04 ± 0.14 Female 22 78.28 ± 30.28 4.53 ± 2.92 11.08 ± 8.01  1.07± 0.13 p-value <0.001  0.493 0.113 0.371 OCP Use (Females Only) Yes 7107.33 ± 27.87  6.27 ± 3.76 12.83 ± 11.64 1.06 ± 0.21 No 15 64.73 ±20.59 3.71 ± 2.12 10.26 ± 5.98  1.07 ± 0.08 p-value <0.001  0.152 0.8410.646 Body Mass Index <25 kg/m² 39 66.0 ± 26.9 4.63 ± 2.49 8.59 ± 5.281.04 ± 0.12 ≧25 kg/m² 10 57.2 ± 25.0 2.42 ± 1.61 13.43 ± 10.19 1.09 ±0.19 p-value 0.284 0.003 0.073 0.438 Exercise ≧120 Minutes/Week 21 73.33± 26.11 4.14 ± 2.56 11.76 ± 8.35  1.09 ± 0.13 <120 Minutes/Week 26 58.66± 27.97 4.20 ± 2.58 8.19 ± 4.85 1.03 ± 0.13 p-value 0.038 0.863 0.0890.165 Race White 31 68.84 ± 28.65 4.94 ± 2.43 7.84 ± 3.92 1.03 ± 0.10Non-White 18 56.30 ± 24.76 2.87 ± 2.04 12.56 ± 9.29  1.08 ± 0.18 p-value0.138 <0.001  0.065 0.346

TABLE 4 Bioavailable 25(OH)D Predicts Bone Mineral Density (BMD).Bioavailable 25(OH)D and BMD were natural log transformed prior toanalysis. The coefficient (B) represents the average unit increase in lnBMD for each unit increase in ln bioavailable 25(OH)D. P-value is thestatistical significance of the relationship between bioavailable25(OH)D and BMD after controlling for potential confounders. Thus, therelationship between bioavailable 25(OH)D and BMD remains significantafter adjusting for potential confounders. Model B P-value Adjusted R²Bioavailable 25(OH)D 0.092 0.002 0.177 Bioavailable 25(OH)D, 0.072 0.0290.180 age, sex, race, and BMI

TABLE 5 Characteristics of the population (n = 94) Median (IQR) or n (%)Age, years 65 (50-74) Male 55 (59%) Black race 48 (51%) Survived atleast one year on dialysis 47 (50%) Body mass index 25 (22-30) Systolicblood pressure, mm Hg 140 (123-153) Diastolic blood pressure, mm Hg 73(61-81) Total 25(OH)D, ng/ml 20 (13-28) Total 1,25(OH)₂D, ng/ml 9.5(5-16) Parathyroid hormone, pg/ml 190 (96-307) Corrected Calcium, mg/dl8.9 (8.5-9.4) Phosphorus, mg/dl 4.2 (3-5.5) Alkaline phosphatase, mg/dl82 (66-112.5) Albumin, g/dl 3.4 (3.0-3.8) Vitamin D binding protein,μg/ml 158 (69-217) Bioavailable 25(OH)D, ng/ml 3.4 (2.2-5.0)Bioavailable 1,25(OH)₂D, pg/ml 2.2 (1.1-3.8)

TABLE 6 Race and vitamin D levels. Black individuals had lower total,but not bioavailable, 25(OH)D levels when compared with their whitecounterparts. Survivors are patients who survived for at least one yearafter initiating hemodialysis, while non-survivors died within thisyear. All values represent group medians. Blacks Whites p Total 25(OH)D(ng/ml) 15.1 23.1 <0.001 Bioavailable 25(OH)D (ng/ml) 3.8 2.8 0.21 Total1,25(OH)₂D (pg/ml) 8 11.5 0.07 Bioavailable 1,25(OH)₂D (pg/ml) 2.2 2.20.48 DBP (μg/ml) 75 189 <0.001 Survivors: DBP (μg/ml) 88 195 0.004Non-survivors: DBP (μg/ml) 58 183 <0.001 PTH (pg/ml) 201 168 0.47

TABLE 7 PTH and bioavailable 25(OH) vitamin D. In univariate andmultivariate analyses, bioavailable 25(OH) vitamin D levels wereconsistently associated with PTH (corresponding p values displayed). PTHand bioavailable vitamin D levels were log transformed prior toanalysis, thus β = −0.36 suggests that a 25% increase in bioavailable25(OH)D is associated with 7.7% decrease in PTH ((1.25^(−0.36) − 1)*100= −7.7). β p Bioavailable 25(OH)D alone −0.36 0.007 Multivariate modeladding age, gender, race −0.33 0.02 Multivariate model with abovevariables plus survival −0.32 0.02 status at 1 year Multivariate modelwith above variables plus calcium, −0.39 0.02 phosphorus, bioavailable1,25(OH)₂D

TABLE 8 Calculation of bioavailable vitamin D levels Number Equation 1[D_(DBP)] = [Total Vitamin D] − [D_(Alb)] − [D_(free)] 2 [D_(Alb)] =K_(alb) · [Alb] · [D_(free)] 3 [D_(free)] = [D_(DBP)] ÷ K_(DBP) ÷[DBP_(free)] 4 [DBP_(free)] = [Total DBP] − [D_(DBP)] From equations 3and 4 5 [D_(free)] = [D_(DBP)] ÷ K_(DBP) ÷ ([Total DBP] − [D_(DBP)])From equations 1 and 2 6 [D_(DBP)] = [Total Vitamin D] − (K_(alb) ·[Alb] + 1) · [D_(free)] 7 [D_(free)] = {[Total Vitamin D] − (K_(alb) ·[Alb] + 1) · [D_(free)]} ÷ K_(DBP) ÷ ([Total DBP] − {[Total Vitamin D] −(K_(alb) · [Alb] + 1) · [D_(free)]}) This can be simplified to fit asecond-degree polynomial (ax² + bx + c = 0) where x = [D_(free)]: a =K_(DBP) · K_(alb) · [Alb] + K_(DBP) b = K_(DBP) · [Total DBP] − K_(DBP)· [Total Vitamin D] + K_(alb) · [Alb] + 1 c = −[Total Vitamin D] 8[D_(free)] = [−b + √b2 − 4ac] ÷ 2a 9 [D_(bioavailable)] = [D_(free)] +[D_(Alb)] = (K_(alb) · [Alb] + 1) · [D_(free)] [Total Vitamin D] = totalmeasured vitamin D concentration (either 25-OH or 1,25-(OH)₂ vitamin D)[Alb] = measured albumin concentration [Total DBP] = measured vitamin Dbinding protein concentration [D_(Alb)] = concentration of albumin-boundvitamin D [D_(DBP)] = concentration of DBP-bound vitamin D [D_(free)] =concentration of free (unbound) D [D_(bioavailable)] = concentration ofBioavailable D = [D_(free)] + [D_(Alb)] K_(alb) = affinity constantbetween vitamin D and albumin = 6 × 10⁵ M⁻¹ (for 25-OH D) or 5.4 × 10⁴M⁻¹ (for 1,25-(OH)₂ D) K_(DBP) = affinity constant between vitamin D andDBF = 7 × 10⁸ M⁻¹ (for 25-OH D) or 3.7 × 10⁷ M⁻¹ (for 1,25-(OH)₂ D)

TABLE 9 Comparison of 25(OH)D insufficiency in blacks vs. whites basedon current definitions % 25(OH)D Deficient 25(OH)D Population Age(years) Blacks Whites Cutoff Levels Reference NHANES 2001-2006 >45(82-85%) 80 40 <20 ng/mL Diaz⁵¹ Older men (MrOS) >65 65 23 <20 ng/mLOrwoll¹⁷ 22 2 <10 ng/mL Hemodialysis 63 ± 15 31 12 <10 ng/mL Wolf⁴³patients (ArMORR) Men in BACH/Bone 30-79 44 11 <21 ng/mL Hannan¹⁹ SurveyNHANES = National Health and Nutrition Examination Survey; MrOS =Osteoporotic Fractures in Men Study; ArMORR = Accelerated Mortality onRenal Replacement; BACH = Boston Area Community Health

Example 5 VDBP Polymorphisms

Vitamin D binding protein (VDBP) is the primary vitamin D carrierprotein, binding 85-90% of total circulating 25(OH)D (Bikle et al., JClin Endocrinol Metab 1986; 63:954-9). VDBP appears to inhibit someactions of vitamin D, as the bound fraction may be unavailable to exertbiological actions on target cells (Safadi et al., J Clin Invest 1999;103:239-51; Chun et al., J Clin Endocrinol Metab 2010; 95:3368-76; Bikleand Gee, Endocrinology 1989; 124:649-54). Common genetic polymorphismsin the VDBP gene track with race (Engelman et al., J Clin EndocrinolMetab 2008; 93:3381-8; Constans et al., Am J Phys Anthropol1985:68:107-22; Lauridsen et al., Calcif Tissue Int 2005; 77:15-22;Lauridsen et al., Clin Chem 2001; 47:753-6.) and produce variant VDBPproteins, which differ in affinity for vitamin D (Chun et al., J ClinEndocrinol Metab 2010; 95:3368-76; Arnaud et al., Hum Genet 1993;92:183-8; Braun et al., Hum Genet 1992; 89:401-6). Clinical assayspresently measure total 25(OH)D without distinguishing fractions boundto major carrier proteins.

We hypothesized that VDBP genotypes might affect circulating VDBPconcentrations and thus account for observed racial differences inmanifestations of vitamin D deficiency.

Participants in the HANDLS study as described in Example 4, weregenotyped for two common single nucleotide polymorphisms in the VDBPgene (rs4588 and rs7041). See the supplementary appendix for detailedmethods. We successfully genotyped 1999 participants.

Methods

Study Population

Healthy Aging in Neighborhoods of Diversity across the Life Span(HANDLS) is a population-based cohort study supported by the IntramuralResearch Program of the National Institute on Aging, National Institutesof Health (N=3720). Study participants were age 30 to 64 years, lived inBaltimore, Md., and were recruited from 13 contiguous U.S. Censustracts. Participants were randomly sampled from within age, race,gender, and socioeconomic status strata, excluding those who did notself-identify as black or white. The Medstar Research Institute'sInstitutional Review Board approved the protocol. The Partners Committeeon Human Research exempted the present study from review.

Data Collection

We used cross-sectional data from HANDLS collected between 2004 and2008. After a home-based interview, participants underwent anexamination on a mobile research vehicle where blood was sampled, heightand weight measured, and bone densitometry performed. Only those whocompleted the examination, including bone densitometry (restricted toweight less than 270 pounds), and who had sufficient blood samplesavailable were included in the present study (N=2085). Bone densitometrywas performed using the Lunar DPX-IQ (Lunar Corp., Madison, Wis.).Femoral neck bone mineral density was examined in detail given itsclinical relevance as a risk factor for hip fracture.

Laboratory Analysis

Blood samples drawn at the examination were frozen for future analysis.Total 25(OH)D₂ and 25(OH)D₃ levels were measured using tandem massspectrometry (inter-assay CV 8.6%; Schottker et al., PLoS One 2012;7:e48774). VDBP levels were measured using a commercial enzyme-linkedimmunosorbant assay (R&D Systems, Minneapolis, Minn., inter-assay CV15%).

Intact parathyroid hormone levels were measured using the Cobaselectrochemiluminescense immunoassay on the Modular Analytics E170automated analyzer (Roche Diagnostics, Indianapolis, Ind., USA;inter-assay CV 2.5%). Calcium levels were corrected for albumin (serumcalcium+0.8*[4-serum albumin]).

Genotyping

Participants were genotyped for two common single nucleotidepolymorphisms in the VDBP gene (rs4588 and rs7041).

VDBP Single Nucleotide Polymorphism Genotyping

Samples were genotyped for two common single nucleotide polymorphisms inthe VDBP gene (rs4588 and rs7041). All samples were genotyped using theABI PRISM 7900HT Sequence Detection System (Applied Biosystems, FosterCity, Calif.), in 384-well format. The 5′ nuclease assay (TaqMan®) wasused to distinguish the 2 alleles of a gene. PCR amplification wascarried out on 5-20 ng DNA using 1× TaqMan® universal PCR master mix (NoAmp-erase UNG) in a 5 μl reaction volume. Amplification conditions on anAB 9700 dual plate thermal cycler (Applied Biosystems, Foster City,Calif.) were as follows: 1 cycle of 95° C. for 10 min, followed by 50cycles of 92° C. for 15 s and 60° C. for 1 min. TaqMan® assays wereordered using the ABI Assays-on-Demand service. The success rate forgenotyping was 95.8%.

HANDLS Whole Genome Sequencying and Ancestry Estimates

Participants were successfully genotyped to 907763 SNPS at theequivalent of Illumina 1M SNP coverage (709 samples using Illumina 1Mand 1Mduo arrays, the remainder using a combination of 550K, 370K, 510Sand 240S to equate the million SNP level of coverage), passing inclusioncriteria into the genetic component of the study. Initial inclusioncriteria for genetic data in HANDLS includes concordance between selfreported sex and sex estimated from X chromosome heterogeneity, >95%call rate per participant (across all equivalent arrays), concordancebetween self-reported African ancestry and ancestry confirmed byanalyses of genotyped SNPs, and no cryptic relatedness to any othersamples at a level of proportional sharing of genotypes>15% (effectivelyexcluding 1^(st) cousins and closer relatives from the set of probandsused in analyses). In addition, SNPs were filtered for HWE p-value>1e-7,missing by haplotype p-values>1 e-7, minor allele frequency>0.01, andcall rate>95%. Basic genotype quality control and data management wasconducted using PLINKv 1.06 (PMID: 17701901). Cryptic relatedness wasestimated via pairwise identity by descent analyses in PLINK andconfirmed using RELPAIR (PMID: 11032786).

Ancestry estimates were assessed using both STRUCTUREv2.3 (PMID:10835412, PMID: 12930761, PMID: 18784791) and the multidimensionalscaling (MDS) function in PLINKv1.06. In the multidimensional scalinganalysis, HANDLS participants were clustered with data made availablefrom HapMap Phase 3 for the YRI, ASW, CEU, TSI, JPT and CHB populations,using a set of 36892 linkage-disequilibrium-pruned SNPs common to eachpopulation. This set of SNPs were chosen as they are not in r²>0.20 withanother SNP in overlapping sliding windows of 100 SNPs in the ASWsamples. HANDLS participants with component vector estimates consistentwith the HapMap ASW samples for the first 4 component vectors wereincluded. In addition, the 1024 quality controlled HANDLS samples werelater clustered among themselves using MDS to generate 10 componentvectors estimating internal population structure within the HANDLSstudy. Of the SNPs utilized for MDS clustering, the 2000 SNPs with themost divergent allele frequency estimates between African populations(frequency estimates based on YRI samples) and European populations(frequency estimates based on combined CEU and TSI samples) wereutilized as ancestry informative markers (AIMs). These 2000 AIMs wereassociated with frequency differences on the level of p-values<1e-3based on chi-squared tests. A two population model in STRUCTURE was usedto estimate percent African and percent European ancestry in the HANDLSsamples, for a 10000 iteration burn-in period, and a 10000 iterationfollow-up of the Markov Chain Monte Carlo model utilized by STRUCTURE.The ancestry estimates from STRUCTURE were highly concordant with thefirst component vector of the MDS clustering of HANDLS samples, with anr² of >0.82.

HANDLS participant genotypes were imputed using MACH and miniMac basedon combined haplotype data for HapMap Phase 2 YRI and CEU samples thatincludes monomorphic SNPs in either of the two constituent populations(release 22, build 36.3). This process followed two stages, firstestimating recombination and crossover events in a random sample of 200participants, then based on this data and the reference haplotypes, 200iterations of the maximum likelihood model were used to estimategenotype dosages for imputed SNPs. After filtering based on a minimumimputation quality of 0.30, indicated by the RSQR estimate in MACH, witha total yield of 2939993 SNPs (note, this data sent to you has beenfiltered for RSQR>0.3 and MAF>0.01). Genotype clusters are available forSNPs genotyped in HANDLS upon request. Data imputed to the entire phase1 alpha version 3 data freeze of the 1000 Genomes Project is availablefor analyses upon request.

We successfully genotyped 1999 participants.

Calculation of Bioavailable 25(OH) D Levels

Bioavailable 25(OH)D was defined as the circulating 25(OH)D not bound toVDBP, analogous to bioavailable testosterone (Vermeulen et al., J ClinEndocrinol Metab 1999; 84:3666-72). Bioavailable 25(OH)D was calculatedin subjects homozygous at both rs7041 and rs4588 (N=1033), for whom wecould use a single genotype-specific binding affinity constant, asfollows (see also Arnaud et al., Hum Genet 1993; 92:183-8):

Definitions

D=25-hydroxyvitamin D (calcidiol), sum of both D2 and D3

Alb=albumin

DBP_(1S)=Gc1S variant of the Vitamin D binding protein, as encoded bythe GC gene containing the rs7041 single nucleotide polymorphisms, witha T>G substitution resulting in the substitution of Aspartic acid withglutamic acid at residue 416 of the VDBP polypeptide. The rs4588 singlenucleotide polymorphism for Gc1S (C) is the ancestral allele encodingthreonine at position 420.

DBP_(1F)=Gc1F variant of the Vitamin D binding protein, as encoded bythe GC gene containing the ancestral alleles for both rs7041 and rs4588single nucleotide polymorphisms; these alleles encode for aspartic acidand threonine at positions 416 and 420 of the VDBP polypeptide.

DBP₂=Gc2 variant of the Vitamin D binding protein, as encoded by the GCgene containing the rs4588 SNP, with a C>A substitution resulting in thesubstitution of threonine acid with lysine at residue 420 of the VDBPpolypeptide. The rs7041 single nucleotide polymorphism for Gc2 is theancestral allele (T) encoding aspartic acid at position 416.

DBP_(1S)=Gc1S variant of the Vitamin D binding protein, as encoded bythe GC gene containing the rs7041 SNP, with a T>G substitution resultingin the substitution of Aspartic acid with glutamic acid at residue 416of the VDBP polypeptide.

-   -   [D_(Alb)]=concentration of albumin-bound vitamin D    -   [D_(DBP)]=concentration of DBP-bound vitamin D    -   [D]=concentration of free (unbound) D    -   [Total]=concentration of Total 250H-D=[D_(DBP)]+[D_(Alb)]+[D]    -   [Bio]=concentration of Bioavailable D (Bioavailable=sum of free        and albumin-bound vitamin)=[D]+[D_(Alb)]    -   K_(alb)=affinity constant between vitamin D and albumin=6×10⁵        M⁻¹    -   KDBP_(generic)=genotype-nonspecific affinity constant between        25-hydroxyvitamin D and DBP=0.7×10⁹ M⁻¹    -   KDBP_(1S)=affinity constant between vitamin D and        DBP_(1S)=0.6×10⁹ M⁻¹    -   KDBP_(1F)=affinity constant between vitamin D and        DBP_(1F)=1.12×10⁹ M⁻¹    -   KDBP₂=affinity constant between vitamin D and DBP₂=0.36×10⁹ M⁻

Equations

Total 25(OH)-Vitamin D

[Total]=concentration of 25(OH)-Vitamin D in g/mol÷400.5 g/mole

Given that [Total]=[D]+[D _(Alb) ]+[D _(DBP)]

thus [D _(DBP)]=[Total]−[D _(Alb) ]−[D]  (Eq. 1)

Albumin

[Alb]=serum albumin concentration in g/L÷66,430 g/mole

[D]+[Alb]·[D _(Alb)]

Albumin association constant K _(alb) =[D _(Alb)]÷([D]·[Alb])

Thus [D _(Alb) ]=K _(alb)·[Alb]·[D]  (Eq. 2)

-   -   (NB: [Alb] in this example denotes the concentration of free        non-vitamin bound albumin. However, given the low affinity        between albumin and Vit. D, the concentrations of total albumin        and unbound albumin are effectively equivalent ([Total        Albumin]≈[Alb]). As a result, [Alb] may be estimated accurately        by measurements of total serum albumin.)

DBP

[Total DBP]=concentration of serum DBP in g/L÷58,000 g/mole

[DBP]=free unbound DBP and [D _(DBP)]=vitamin-bound DBP

Given that [D]+[DBP]

[D _(DBP)]

And DBP association constant K _(DBP) =[D _(DBP)]÷([DBP]·[D])

Thus [D]=[D _(DBP) ]÷K _(DBP)÷[DBP]  (Eq. 3)

Since [Total DBP]=sum of bound and unbound DBP=[DBP]+[D _(DBP)]

Therefore [DBP]=[Total DBP]−[D _(DBP)]  (Eq. 4)

Solving for Free 25(OH)-Vitamin D

From Eqs. 3 and 4 we see that:

[D]=[D _(DBP) ]÷K _(DBP)÷([Total DBP]−[D _(DBP)])  (Eq. 5)

If we substitute Eq. 1 into Eq. 2, we find that:

[D _(DBP)]=[Total]−(K _(alb)·[Alb]+1)·[D]  (Eq. 6)

Substituting Eq. 6 into Eq. 5 produces the following:

[D]=([Total]−(K _(alb)·[Alb]+1)·[D])÷K _(DBP)÷([Total DBP]−([Total]−(K_(alb)·[Alb]+1)·[D]))

The equation is now limited to known constants (K_(DBP) and K_(alb)),measured values ([Total DBP], [Alb], and [Total]) and the dependentvariable for free vitamin D [D]. After propagating products and severalrearrangements we can further simplify this to fit the form of asecond-degree polynomial:

ax ² +bx+c=0

-   -   Where x=[D]=the concentration of free 25(OH)-Vitamin D

a=K _(DBP) ·K _(alb)·[Alb]+K_(DBP)

b=K _(DBP)·[Total DBP]−K _(DBP)·[Total]+K _(alb)·[Alb]+1

c=−[Total]

-   -   This polynomial may be solved for [D] using the quadratic        equation:

$\lbrack D\rbrack = \frac{{- b} + \sqrt{b^{2} - {4a\; c}}}{2a}$

-   -   After solving for free 25(OH)-vitamin D, we may then use Eq. 2        to calculate the concentration of bioavailable (non-DBP bound        vitamin):

[Bio]=[D]+[D _(alb)]=(K _(alb)·[Alb]+1)·[D]  (Eq. 7)

-   -   Furthermore, if the DBP genotype for an individual patient is        known, for patients who are homozygous for Gc1S/Gc1S, Gc1F/Gc1F,        or Gc2/Gc2, the genotype-adjusted free and bioavailable        fractions of 25-hydroxyvitamin D can be calculated using the        known binding affinities for the three variants (ref. 34):    -   For subjects homozygous for Gc1F variant, K_(DBP)=1.12×10⁸M⁻¹    -   For subjects homozygous for Gc1S variant, K_(DBP)=0.60×10⁸M⁻¹    -   For subjects homozygous for Gc2 variant, K_(DBP)=0.36×10⁸M⁻¹

Example Calculations

For a patient with a known VDBP genotype indicating homozygosity forGc1F/Gc1F:

Total 25(OH)-vitamin D=[Total]=40 ng/mL=1.0×10⁻⁷ mol/L

Total serum DBP=[Total DBP]=250 ug/mL=4.3×10⁻⁶ mol/L

Total serum albumin=[Alb]=4.3 g/dL=6.4×10⁻⁴ mol/L

K _(alb)=6×10⁵ M⁻¹

K _(DBP)=1.12×10⁸M⁻¹

a=4.36×10¹¹

b=5147

c=−1×10⁻⁷

Calculated concentration of free 25(OH)D=1.94×10⁻¹¹ mol/L=7.8 pg/mLCalculated concentration of bioavailable 25(OH)D=7.54×10⁻⁹ mol/L=3.0ng/mL

Genotype-specific calculated bioavailable 25-hydroxyvitamin Dconcentrations were validated by directly measuring non-VDBP-bound25-hydroxyvitamin D in a subset of homozygous subjects using acompetitive radioligand binding assay (see Example 5).

Statistical Analysis

Race-stratified participant characteristics are presented asmeans±standard errors or number (percent) and compared using t-tests forcontinuous variables and chi-square tests for categorical variables.Non-normally distributed variables were natural log transformed to meetassumptions of parametric testing. Adjusted means and standard errorswere derived from multiple linear regression models containing terms forage, sex, body mass index, poverty status, season, smoking status, andcalcium intake. For models examining racial differences, interactionsbetween race and the main predictor of interest were included ascovariates.

Subjects were divided into quintiles to examine relationships between25(OH)D measures and plasma parathyroid hormone, serum calcium, and bonemineral density. For quintile analyses, P-values are presented from bothcategorical and continuous models. R-squared values for models exploringvariation in total 25(OH)D and VDBP levels were derived fromsemi-partial correlation coefficients using Type I sums of squares frommultiple linear regression models.

Chi-squared tests were used to compare allele frequencies in blacks andwhites. Unadjusted, race-stratified, additive effects models were usedto summarize associations between VDBP and 25(OH)D levels and the twosingle nucleotide polymorphisms of interest (rs7041 and rs4588). In asubset of 783 samples from black participants with complete data fromGenome-Wide Association Studies, identical statistical models were rerunusing 10 principal components or genome-wide percent African ancestryestimates as covariates. The parameters adjusting for populationsubstructure were not significant and made little impact on the model.Therefore, we did not include these covariates in reported models.

Statistical analyses were conducted in SAS version 9.2. Two-tailedP-values of less than 0.05 were considered significant with theexception of genotype analysis where p-values were adjusted for thepresence of 2 SNPs (P<0.025).

Results

Participant Characteristics

Black (N=1181) and white (N=904) participants were similar in age, sex,body mass index, and female menopausal status (Table 10). Blacks weremore likely than whites to be impoverished, active smokers, or havemicroalbuminuria. Use of hormone replacement therapy and medicationsthat affect vitamin D metabolism was uncommon.

TABLE 10 Characteristics of Community-Dwelling Blackand White AmericanParticipants. Overall Black White Characteristic (N = 2,085) (N = 1,181)(N = 904) P-value Male - no. (%) 921 (44.2) 523 (44.3) 398 (44.0) 0.91Age (years) 48.3 ± 0.2 48.3 ± 0.3 48.3 ± 0.3 0.92 Below Poverty Line -no. (%) 850 (40.8) 573 (48.5) 277 (30.6)  <0.001* Body Mass Index(kg/m²) 29.6 ± 0.2 29.4 ± 0.2 29.8 ± 0.2 0.14 Houston Activity Scale† 2.4 ± 0.1  2.2 ± 0.1  2.8 ± 0.1  <0.001* Current Smoker - no. (%) 930(44.6) 552 (46.7) 378 (41.8)  0.01* Hx of Osteoporosis - no. (%) 51(2.5) 19 (1.6) 32 (3.5)  0.01* Prescribed Osteoperosis Therapies - 29(1.4) 10 (0.9) 19 (2.1)  0.02* no. (%)†† Post-Menopausal - no. (%) of823 (53.8) 345 (52.8) 278 (55.1) 0.42 Females HRT - no. (%) of Females27 (2.4) 10 (1.6) 17 (3.5)  0.045* Urine Microalbumin > 30 (mg/dl)- no.37 (1.8) 27 (2.3) 10 (1.1)  0.01* (%) eGFR by CKD-EPI < 60 (mL/min/1.73114 (5.5)  67 (5.7) 47 (5.2) 0.53 m²) - no. (%) PrescribedAnti-Epileptics - no. (%)†† 14 (0.7)  6 (0.5)  8 (0.9) 0.30 PrescribedCorticosteroids - no. (%)†† 28 (13)  20 (1.7)  8 (0.9) 0.11 Vitamin DIntake (IU/day) 152 ± 4  149 ± 5  157 ± 6  0.38 Calcium Intake (mg/day)731 ± 11 720 ± 14 744 ± 17 0.10 Hx = History, HRT = Hormone ReplacementTherapy, eGFR = Estimated Glomerular Filtration Rate †One-thousand,four-hundred, and fifty-seven participants had missing data (69.9%)††Anti-epileptics include: Phenobarbital, Carbamazepine, Phenytoin,Primidone; Corticosteroids include: Prednisone, Hydrocortisone,Methylprendisolone, Prednisolone, Dexamethasone; Osteoperosis therapiesinclude: Pamidronate, Neridronate, Olpadronate, Alendronate,Ibandronate, Risedronate, Zoledronate, Denosumab, Teraparatide, andRaloxifene *Significant at P < 0.05

Total 25(OH) D and VDBP Levels

Unadjusted total 25(OH)D levels were lower in blacks compared to whites(15.6±0.2 ng per ml versus 25.8±0.4 ng per ml, P<0.001). Racialdifferences in total 25(OH)D levels persisted after multivariateadjustment, including adjustment for microalbuminuria (17.3±0.3 ng perml in blacks versus 25.5±±0.4 ng per ml in whites, P<0.001). There wereseasonal differences in 25(OH)D (Table 11). In a model without thegenetic polymorphisms of interest, race explained 16.3% of the variationin total 25(OH)D levels.

Unadjusted VDBP levels were lower in black compared to whites (168±3versus 337±5 μg/mL, P<0.001). Racial differences in VDBP persisted aftermultivariate adjustment, including adjustment for microalbuminuria,(169±5 in blacks versus 337±5 in whites, P<0.001). There were seasonaldifferences in VDBP levels (Table 11). In a model without geneticpolymorphisms of interest, race explained 30.2% of the variation in VDBPlevels.

TABLE 11 Seasonal Differences Summer Fall Spring Winter Blacks no. (%)182 (15.4) 321 (27.2) 353 (29.9) 325 (27.5) Total 25(OH)D (ng/ml) 19.0 ±0.6** 18.1 ± 0.4** 13.2 ± 0.4 13.8 ± 0.4 VDBP (μg/ml) 173 ± 9   167 ±6    153 ± 6** 183 ± 6  PTH (pg/ml) 35.8 ± 1.0** 36.3 ± 1.0** 40.7 ± 1.041.6 ± 1.0 Calcium (mg/dl)  9.13 ± 0.03** 9.14 ± 0.2**   9.3 ± 0.02** 9.04 ± 0.02 Whites no. (%) 223 (24.7) 278 (30.8) 202 (22.4) 201 (22.2)Total 25(OH)D (ng/ml) 30.5 ± 0.7** 27.4 ± 0.6** 22.1 ± 0.7 22.0 ± 0.7VDBP (μg/ml) 353 ± 9*  324 ± 9    353 ± 10* 322 ± 10 PTH (pg/ml) 32.9 ±1.0  33.7 ± 1.0  34.1 ± 1.0 34.0 ± 1.0 Calcium (mg/dl) 9.00 ± 0.02  9.02± 0.02*  8.97 ± 0.02  8.94 ± 0.02 25(OH)D = 25-hydroxyvitamin D, VDBP =Vitamin D Binding Protein, PTH = Parathyroid Hormone *Significantlydifferent from winter at P < 0.05, **Significantly different from winterat P < 0.01, †Corrected for albumin

VDBP Genetic Polymorphisms

At rs7041, blacks were more likely to have the A allele, while whiteswere more likely to have the C allele (A allele frequency-0.83 in blacksversus 0.42 in whites, P<0.001). Blacks were less likely to have the Tallele at rs4588 compared to whites (T allele frequency=0.10 in blacksversus 0.28 in whites P<0.001). No subjects had both the C allele atrs7041 and T allele at rs4588.

The A allele at rs7041 resulted in low levels of VDBP in both black andwhite Americans (Table 12). The T at rs4588 resulted in higher VDBP inboth blacks and whites, after accounting for the observation that all ofthese subjects had the A allele at rs7041. The polymorphisms at rs7041and rs4588 both showed dose-dependent effects on VDBP concentrations,with heterozygous subjects showing VDBP concentrations intermediatebetween that of the homozygotes (Table 12). Genetic variantsindependently explained 79% of variation in VDBP levels after accountingfor other factors. After accounting for genetic variants, race explainedless than 1% of variation in VDBP levels.

The A allele at rs7041 was associated with low total 25(OH)D levelsamong blacks. In whites, the T allele at rs4588 was associated withlower total 25(OH)D levels (Table 12). Rs7041 and rs4588 polymorphismsexplained 10.0% of variation in total 25(OH)D levels after accountingfor other factors. In the same model, season, race, and body mass indexexplained 13.5%, 7.3%, and 1.4% of variation in total 25(OH)D levelsrespectively. Sex, age, smoking, calcium intake, body mass index, andmicroalbuminuria each explained less than 2% of variation. Overall 30.2%of variation was explained in a model containing the aforementionedvariables. VDBP concentration independently explained a similar amountof variation as VDBP genotype.

TABLE 12 Influence of Genetic Polymorphisms on Vitamin D Binding Proteinand Total 25-Hydroxyvitamin D Levels Change in Total Change in 25(OH)DVDBP level level per Variant per variant variant Reference VariantAllele allele copy allele copy Race SNP Allele Allele Frequency N (95%CI) P-value (95% CI) P-value Black rs7041 C A 0.83 1,137 −189.4 (−195.6,<0.0001* −1.9 (−2.8, <0.0001* −183.1) ug/ml −1.0) ng/ml rs4588 G T 0.1057.2 (49.5, <0.0001* −0.6 (−1.7, 0.29 64.9) ug/ml 0.5) ng/ml Whiters7041 C A 0.42 862 −189.1 (−201.0, <0.0001* 0.2 (−1.3, 0.80 −177.2)ug/ml 1.7) ng/ml rs4588 G T 0.28 49.9 (36.9, <0.0001* −2.5 (−4.1, 0.002*62.98) ug /ml −0.9) ng/ml 25(OH)D = Total 25-hydroxyvitamin D, VDBP =Vitamin D Binding Protein Significant at P < 0.025 (adjusted for 2 SNPs)

Bioavailable 25(OH)D in Homozygous Subjects

FIG. 9A shows the percentage of homozygous participants in each racialgroup with each variant VDBP protein (resulting from unique combinationsof the rs7041 and rs4588 polymorphisms). We calculated theconcentrations of bioavailable 25(OH)D based upon plasma concentrationsof total 25(OH)D, VDBP, and the specific 25(OH)D binding affinity ofeach VDBP variant. Estimated bioavailable 25(OH)D levels were similar in1033 black and white homozygous subjects (2.9±0.1 ng/ml in blacks versus3.1±0.1 ng/ml in whites, P=0.71, FIG. 9B).

Levels of 25(OH)D and Markers of Vitamin D Status

Adjusted mean femoral neck bone mineral density was greater in blacksthan in whites (1.05±0.01 versus 0.94±0.01 g/cm², P<0.001), as wereadjusted mean calcium levels (9.11±0.01 versus 8.99±0.01 mg/dl,P<0.001). Femoral neck bone mineral density was not associated withbioavailable or total 25(OH)D in black participants; however, in whites,bone mineral density increased with increasing total or bioavailable25(OH)D levels (Table 13). Calcium levels increased with increasingtotal 25(OH)D in blacks only (Table 13).

Adjusted mean levels of parathyroid hormone were greater in blacks thanin whites (39±1 pg/mL versus 34±1, P<0.001). Lower total or bioavailable25(OH)D levels were associated with higher levels of parathyroid hormonein both races (Table 13). When compared to whites with similarparathyroid hormone levels, blacks had total 25(OH)D levels that weresignificantly lower. In contrast, homozygous black and white Americanswith similar parathyroid hormone levels had similar bioavailable 25(OH)Dlevels.

TABLE 13 Total and Bioavailable 25(OH)D and Markers of Vitamin DSufficiency in Black and White Americans Total Bioavailable 25(OH) D BMDPTH Calcium 25(OH) D BMD PTH Calcium Quintile Mean ± Mean ± Mean ±Quintile Mean ± Mean ± Mean ± (Min-Max) N SE^(†) SE^(†) SE^(†) (Min-Max)N SE^(†) SE^(†) SE^(†) Black Black 1 (0-10) 357 1.05 ± 43.3 ± 9.08 ± 1(0.00-1.40) 131 1.03 ± 44.3 ± 9.07± 0.01 1.0 0.02 0.02 1.0 0.03 2(11-15) 324 1.06 ± 38.1 ± 9.08 ± 2 (1.41-1.99) 138 1.05 ± 41.4 ± 9.09 ±0.01 1.0 0.02 0.01 1.0 0.03 3 (16-20) 221 1.05 ± 36.8 ± 9.14 ± 3(2.00-2.77) 112 1.05 ± 38.1 ± 9.17 ± 0.01 1.0 0.03 0.02 1.0 0.04 4(21-28) 193 1.05 ± 36.5 ± 9.10 ± 4 (2.78-4.24) 139 1.03 ± 36.2 ± 9.11 ±0.01 1.0 0.03 0.01 1.0 0.03 5 (29-90)  86 1.03 ± 33.1 ± 9.21 ± 5(4.25-15.72) 126 1.06 ± 34.1 ± 9.11 ± 0.02 1.1 0.04 0.02 1.0 0.04P-value for Trend 0.53 <0.001* 0.02* P-value for Trend 0.53 <0.001* 0.34White White 1 (0-10)  53 0.92 ± 41.5 ± 8.95 ± 1 (0.00-1.40) 71 0.88 ±39.3 ± 9.06 ± 0.02 1.1 0.05 0.02 1.0 0.04 2 (11-15) 101 0.90 ± 38.2 ±8.99 ± 2 (1.41-1.99) 65 0.92 ± 35.4 ± 898± 0.02 1.0 0.03 0.02 1.0 0.04 3(16-20) 165 0.92 ± 35.9 ± 9.00 ± 3 (2.00-2.77) 90 0.97 ± 32.5 ± 9.00 ±0.01 1.0 0.03 0.02 1.0 0.03 4 (21-28) 245 0.94 ± 32.7 ± 8.96 ± 4(2.78-4.24) 64 0.97 ± 29.6 ± 9.03 ± 0.01 1.0 0.02 0.02 1.0 0.04 5(29-90) 340 0.96 ± 30.9 ± 9.01 ± 5 (4.25-15.72) 76 0.95 ± 29.7 ± 9.02 ±0.01 1.0 0.02 0.02 1.0 0.04 P-value for Trend 0.003* <0.001* 0.40P-value for Trend 0.007* <0.001* 0.83 BMD = Bone Mineral Density, PTH =Parathyroid Hormone *Significant at p < 0.05 ^(†)Adjusted for age,season, sex, body mass index, smoking status, socioeconomic status, andcalcium intake (except where calcium is the outcome)

Total 25(OH)D levels are partially genetically determined (Engelman etal., J Clin Endocrinol Metab 2008; 93:3381-8; Hunter et al., J BoneMiner Res 2001; 16:371-8). In the present study, genetic polymorphismsin VDBP explained a greater proportion of variation in total 25(OH)Dthan genetic variants identified in a recent genome wide associationstudy (Wang et al., Lancet 2010; 376:180-8) and a greater proportion ofvariation than most factors known to be associated with vitamin Dlevels. To our knowledge, this is the largest study of VDBPpolymorphisms in a black American population (Wang et al., Lancet 2010;376:180-8; Carpenter et al., J Bone Miner Res 2012; et al., Am J ClinNutr 2009; 89:634-40; Fang et al., Calcif Tissue Int 2009; 85:85-93;Al-oanzi et al., Osteoporos Int 2008; 19:951-60). VDBP only partiallyexplained racial differences total 25(OH)D levels; other factors,including skin pigmentation, contribute to lower total 25(OH)D levels inblacks (Aloia et al., Am J Clin Nutr 2008; 88:545S-50S; Harris S S,Dawson-Hughes et al., J Clin Endocrinol Metab 2007; 92:3155-7; Clemenset al., Lancet 1982; 1:74-6; Bell et al., J Clin Invest 1985; 76:470-3).The effect of VDBP polymorphisms on total 25(OH)D concentrations waslikely mediated through VDBP concentration, as VDBP levels explained asimilar amount of variation as the VDBP polymorphisms. Though theVDBP-null mice require less total circulating 25(OH)D for sufficiency,they are more susceptible to vitamin D deficiency when deprived ofvitamin D; VDBP serves as a 25(OH)D reservoir and aids in thereabsorption of filtered vitamin D through megalin in the kidney (Safadiet al., J Clin Invest 1999; 103:239-51; Nykjaer et al., Cell 1999;96:507-15). We speculate that low VDBP levels may predispose to vitaminD deficiency when vitamin D sources are scarce.

Currently, screening for vitamin D deficiency with total 25(OH)D levelsis widespread. Black Americans are frequently classified as vitamin Ddeficient and are supplemented with exogenous vitamin D. Vitamin Ddeficiency is certainly present in those individuals with low total25(OH)D levels accompanied by hyperparathyroidism, hypocalcemia, or lowbone mineral density. However, community dwelling blacks whose total25(OH)D levels are below the threshold used to define vitamin Ddeficiency typically lack the physiologic alterations observed in thiscondition. The high prevalence of a polymorphism in the VDBP geneassociated with low VDBP among blacks maintains similar bioavailable25(OH)D levels to whites despite lower total 25(OH)D levels (FIG. 9C).Alterations in VDBP levels may therefore be responsible for observedracial differences in total 25(OH)D levels and manifestations of vitaminD deficiency. Improved assessment of vitamin D status in diversepopulations will require accounting for VDBP.

Example 6 Validation of Calculated Bioavailable 25(OH)D MeasurementsCompared to Direct Measurement of Bioavailable 25(OH)D Using a25-Hydroxyvitamin D Radioligand Competitive Binding Assay (VRCBA)

This example describes a differential affinity precipitation method,which was used to directly measure non-VDBP-bound 25-hydroxyvitamin D.Genotype-specific calculated bioavailable 25-hydroxyvitamin Dconcentrations as described in Example 5, above, were validated usingthis method in a subset of homozygous subjects using a competitiveradioligand binding assay. Measured and calculated 25-hydroxyvitamin Dwere correlated (In 13 Gc1S homozygotes: R=0.81, in 33 Gc1F homozygotes:R=0.89, P<0.001 for both, FIGS. 10-12).

Materials

25(OH)D affinity adsorption plates were made using Costar 96-well flatbottom EIA plates. Wells were first coated with purified Vitamin DBinding Protein (Gc globulin from human plasma, >90% pure, SigmaAldrich). Each well was treated with 1 microgram of Gc globulin dilutedinto 100 microliters of water and allowed to adsorb at 4° C. overnight.Wells were then washed with water and blocked with 500 microliters humanserum albumin diluted in water (1% w/v). Plates were stored at 4° C.until use, at which time blocking solution was washed away with water.

VDBP calibrators were made by diluting varying concentrations of VitaminD Binding Protein (Gc globulin from human plasma, >90% pure, SigmaAldrich) into a matrix consisting of 125 mM sodium chloride, 25 mMsodium bicarbonate (pH 7.4), and human serum albumin (5% w/v).

Radiolabeled 25-hydroxyvitamin D3 ligand was purchased from Perkin Elmer(Hydroxyvitamin D3, 25-[26,27-3H]-, 5 μCi(185 kBq), Product number:NET349005UC). Radioligand shipped in toluene was dried under a stream ofargon and redissolved in acetonitrile. For each binding assay 1microliter (˜1 nCi) of radioligand was diluted into 100 microliters ofbinding assay buffer (5% human serum albumin diluted 1:5000 in phosphatebuffered saline).

Scintillation counting was performed by mixing all 200 microliters ofacetonitrile-extracted adsorbed radioligand or 200 ul of non-adsorbedradioligand into 3 milliliters of Ultima Gold scintillation fluid(Perkin Elmer). Radioactivity was quantified by measuring scintillationdecays per minute (DPM) on a Packard TriCarb scintillation counter.

Assay Principal

In this microtiter plate-based competitive binding assay, radiolabeled25(OH)D3 partitions between VDBP protein adsorbed to the sides of themicrotiter plate wells and the VDBP protein within patients' dilutedplasma. After binding equilibrium is achieved, the soluble ligand withinthe reaction buffer is removed and the bound ligand is extracted usingacetonitrile. The proportions of ligand that are soluble versus adsorbedare then quantified by scintillation counting. The proportion ofadsorbed ligand relative to total ligand is representative of the amountof vitamin D that is bioavailable (i.e. the fraction not bound topatients' VDBP; see FIG. 10). Thus:

VRCBA % bioavailable 25(OH)D=adsorbed radiolabel÷[adsorbedradiolabel+non-adsorbed]

In order to relate the proportions of adsorbed vs. soluble radioligandto the concentrations obtained using our calculated bioavailable assaymethod, purified VDBP (Gc globulin) was used, diluted at variousconcentrations into a fixed concentration of human serum albumin as aVDBP calibrator solution. Using these calibrator solutions, calibrationcurves were generated and used these to transform VRCBA % bioavailable25(OH)D radioligand measurements in patients samples into theircorresponding calculated % bioavailable 25(OH)D values (FIG. 11). Theabsolute concentration of bioavailable 25(OH)D3 in the patient's plasmawas then obtained by multiplying the total concentration of 25(OH)D3 bythe % bioavailable.

Assay Procedure

Three microliters of patient plasma was diluted into 15 mL of phosphatebuffered saline (1:5000). 100 microliters of diluted plasma was added toeach well. Plates were chilled on ice in a refrigerated room kept at 4°C. for 15 minutes prior to adding radioligand. 100 microliters ofradioligand diluted in binding assay buffer was added to each well andplates were kept refrigerated at 4° C. for 8 hours for binding reactionto reach equilibrium. Soluble radioligand (bound to serum VDBP andalbumin from test samples) was separated from adsorbed ligand bypipetting all 200 microliters of the reaction volume from the wellwithout leaving any visible amounts. This fraction was added directly to3 ml of scintillation fluid. Adsorbed radioligand (bound to purifiedVDBP and albumin coating microtiter wells) was extracted by adding 200microliters of acetonitrile. Plate was tipped back and forth to extractany ligand on upper sides of well, and acetonitrile extract was removedand mixed in 3 ml of scintillation fluid. Scintillation vials were,capped, mixed thoroughly, and counted for 2 minutes each. All samplesand assay calibrator standards were measured in triplicate. Patientsample measurements were performed in two experiments, Each experimentincluded measurement of five assay standards containing VDBP calibratorsat 1000, 500, 250, 125, and 62.5 micrograms per milliliter. Patientsample measurements of % bioavailable radioligand (defined by the ratioof adsorbed radioligand divided by total radioligand) were convertedinto their respective calculated % bioavailable 25(OH)D using thecalibrator standard curves (FIG. 12). The concentration of bioavailable25(OH)D (in ng/ml) in patient samples was obtained by multiplying theirmeasured total 25(OH)D concentrations by the calculated % bioavailable25(OH)D.

Results

Measured and calculated 25-hydroxyvitamin D were correlated (In 13 Gc1Shomozygotes: R=0.81, in 33 Gc1F homozygotes: R=0.89, P<0.001 for both,FIGS. 10-12).

Example 7 Measurement of Serum Bioavailable 25-Hydroxyvitamin D UsingImmobilized DBP Affinity Extraction

This example describes the measurement of serum bioavailable25-hydroxyvitamin D using an exemplary immobilized DBP affinityextraction assay. Samples included (1) 5% human albumin mixed with 100ng/ml 25-hydroxyvitamin D in a Tris binding buffer (25 mM Tris-HCl pH7.4, 150 mM NaCl), (2) The same 5% human albumin spiked with 300micrograms/mL of purified Gc globulin (Sigma Aldrich), (3) de-identifiedpatient serum containing 60 ng/ml of 25-hydroxyvitamin D, and (4) thesame patent's serum spiked with 300 micrograms/mL purified Gc globulin.100 microliter aliquots of the samples were diluted 1:10 with Trisbinding buffer and mixed with 250 microliters of binding resin. Theaffinity binding resin consisted of Pierce anti-FLAG agarose resincoated with immunoadsorbed FLAG-tagged Gc1S protein. The adsorbed Gc1Sprotein was produced by transiently transfecting 293T HEK cells with aGc1S expression vector, and the Gc1S was purified from the cellsupernatant by immunoadsorption to the anti-FLAG agarose. After 1:10dilution, diluted samples were mixed with 250 microliters of resin(coated with 10 micrograms of Gc1S) and incubated at 37 degrees for 15minutes. The resin was spun down, washed with Tris binding buffer, mixedwith isotopic d6-25-hydroxyvitamin D3 internal standard, and the bound25D and internal standard were extracted with glycine pH 2.5 followed byheptane liquid-liquid extraction. The extracts and calibrators weredried, derivatized with 4-Phenyl-1,2,4-triazole-3,5-dione, and analyzedby LC-MS/MS. 25-hydroxyvitamin D levels were normalized to the addedinternal standard values to adjust for differences in yield, andabsolute amounts of extracted 25-hydroxyvitamin D were calculated basedon the calibration curve.

The results, shown in FIG. 15, demonstrated thatbioavailable-hydroxyvitamin D may be affinity extracted from serum anddetected in this manner, and that the affinity-extractable bioavailable25-hydroxyvitamin D decreases with increasing concentrations of DBP, aswould be predicted. Each reaction was performed in triplicate.

SEQUENCE LISTINGSEQ ID NO: 01 Human VDBP Isoform 1, variant 1 NCBI Ref: NP_000574  1 mkrvlvllla vafghalerg rdyeknkvck efshlgkedf tsisivlysr kfpsgtfeqv 61 sqlvkevvsl teaccaegad pdcydtrtsa lsakscesns pfpvhpgtae cctkeglerk121 lcmaalkhqp qefptyvept ndeiceafrk dpkeyanqfm weystnygqa plsllvsytk181 sylsmvgscc tsasptvcfl kerlqlkhis llttlsnrvc sqyaaygekk srlsnlikla241 qkvptadled vlplaeditn ilskccesas edcmakelpe htvklcdnls tknskfedcc301 qektamdvfv ctyfmpaaql pelpdvelpt nkdvcdpgnt kvmdkytfel srrthlpevf361 lskvleptlk slgeccdved sttcfnakgp llkkelssfi dkgqelcady sentfteykk421 klaerlkakl pdatptelak lvnkhsdfas nccsinsppl ycdseidael knilSEQ ID NO: 02 Human VDBP Isoform 1, variant 2 NCBI Ref: NP_001191235  1 mkrvlvllla vafghalerg rdyeknkvck efshlqkedf tslslvlysr kfpsqtfeqv 61 sqlvkevvsl teaccaegad pdcydtrtsa lsakscesns pfpvhpgtae cctkeglerk121 lcmaalknqp gefptyvept ndeiceafrk dpkeyanqfm wevstnyqqa plsllvsytk181 sylsmvgscc tsasptvcfl kerlqlkhls llttlsnrvc sqyaaygekk srlsnlikla241 qkvptadled vlplaeditn ilskccesas edcmakelpe ntvklcdlis tknskfedcc301 qektamdvfv ctyfmpaaql pelpdvelpt nkdvcdpgnt kvmdkytfel srrthlpevf361 lekvieptlk slgeccdved sttcfnakqp llkkeissfi dkggeicady sentfteykk421 klaerlkakl pdatptelak lvnkhsdfas nocsinsppl ycdseidael knilSEQ ID NO: 03 Human VDBP Isoform 2, NCBI Ref: NP_001191236  1 mlwswseerg gaarlsgrkm krvlvlllav afghalergr dyeknkycke fshlgkedft 61 slslvlysrk fpsqtfeqvs qlvkevvslt eaccaegadp dcydtrtsal sakscesnsp121 fpvbpqtaec ctkeglerkl cmaalkhqpq efptyveptn deiceafrkd pkeyanqfmw181 eystnygqap lsllvsytks ylsmvascct sasptvcflk erlqlkhlsl lttlsnrves241 qyaaygekks rlsnliklaq kvptadledv lplaeditni lskccesase dcmakelpeh301 tvklcdnlst knskfedccq ektamdvfvc tyfmpaaqlp elpdvelptn kdvcdpgntk361 vmdkytfels rrthlpevfl skyleptlks lgeccdveds ttcfnakgpl lkkelssfid421 kgqelcadys entfteykkk laerlkaklp datptelakl vnkhsdfasn ccsinspply481 cdseidaelk nil SEQ ID NO: 04 Human preproalbumin NCBI Ref: NP_000468  1 mkwvtfisll flfssaysrg vfrrdahkse vahrfkdlge enfkalvlia faqylqqcpf 61 edhvklvnev tefaktcvad esaencdksl htlfgdklct vatlretyge madccakqep121 ernecflqhk ddnpnlprlv rpevdvmcta fhdneetflk kylyeiarrh pyfyapellf181 fakrykaaft eccqaadkaa cllpkldelr degkassakq rlkcaslqkf gerafkawav241 arlsqrfpka efaevsklvt dltkvhtecc hqdllecadd radlakyice nqdsissklk301 eccekpllek shciaevend empadlpsla adfveskdvc knyaeakdvf lgmflyeyar361 rhpdvsvvll lrlaktyett lekccaaadp hecyakvfde fkplveepqn likqncelfe421 qlgeykfgna llvrytkkvp qvstptlvev srnlgkvgsk cckhpeakrm pcaedylsvv481 lnqlcvlhek tpvsdrvtkc cteslvnrrp cfsalevdet yvpkefnaet ftfhadictl541 sekerqikkq talvelvkhk pkatkeqlka vmddfaafve kcckaddket cfaeegkklv601 aasqaalql SEQ ID NO: 05 Human proalbumin rg vfrrdahkse vahrfkdlge enfkalvlia faqylqqcpfedhvklvnev tefaktcvad esaencdksl htlfgdklct vatlretyge madccakqepernecflqhk ddnpnlprlv rpevdvmcta fhdneetflk kylyeiarrh pyfyapellffakrykaaft eccqaadkaa cllpkldelr degkassakq rlkcaslgkf gerafkawavarlsqrfpka efaevsklvt dltkvhtecc hgdllecadd radlakyice ngdsissklkeccekpllek shciaevend empadlpsla adfveskdvc knyaeakdvf lgmflyeyarrhpdysvvll lrlaktyett lekccaaadp hecyakvfde fkplveepqn likqncelfeqlgeykfqna llvrytkkvp qvstptlvev srnlgkvgsk cckhpeakrm pcaedylsvvlnqlcvlhek tpvsdrvtkc cteslvnrrp cfsalevdet yvpkefnaet ftfhadictlsekerqikkq talvelvkhk pkatkeqlka vmddfaafve kcckaddket cfaeegkklvaasqaalgl SEQ ID NO: 06 Human albumindahkse vahrfkdlge enfkalvlia faqylqqcpfedhvklvnev tefaktcvad esaencdksl htlfgdklct vatlretyge madccakqepernecflqhk ddnpnlprlv rpevdvmcta fhdneetflk kylyeiarrh pyfyapellffakrykaaft eccqaadkaa cllpkldelr degkassakq rlkcaslqkf gerafkawavarlsqrfpka efaevsklvt dltkvhtecc hgdllecadd radlakyice nqdsissklkeccekpllek shciaevend empadlpsla adfveskdve knyaeakdvf lgmflyeyarrhpdysvvll lrlaktyett lekccaaadp hecyakvfde fkplveepqn likqncelfeqlgeykfqna llvrytkkvp qvstptlvev srnlgkvgsk cckhpeakrm pcaedylsvvlnqlcvlhek tpvsdrvtkc cteslvnrrp cfsalevdet yvpkefnaet ftfhadictlsekerqikkq talvelvkhk pkatkeqlka vmddfaafve kcckaddket cfaeegkklvaasqaalgl SEQUENCE SEQ ID NO:AGCAAAATTGCCTGATGCCACACCCA[A/C]GGAACTGGCAAAGCTGGTTAACAAG 7GAGCGACTAAAAGCAAAATTGCCTGA[G/T]GCCACACCCACGGAACTGGCAAAGC 8

1. A method for treating a Vitamin D insufficiency in a subject, the method comprising: determining a level of bioavailable Vitamin D in the subject by directly detecting levels of free Vitamin D and Vitamin D bound to albumin in a sample comprising serum or plasma from the subject using a differential affinity precipitation assay; comparing the level of bioavailable Vitamin D in the sample to a reference level of Vitamin D; identifying a subject who has a level of bioavailable Vitamin D below the reference level of Vitamin D as having a Vitamin D insufficiency; and administering a vitamin D insufficiency treatment to a subject identified as having a vitamin D insufficiency.
 2. The method of claim 1, wherein the differential affinity precipitation assay is performed by a method comprising: contacting a sample comprising serum or plasma from the subject with purified Vitamin D Binding Polypeptide (VDBP), wherein the purified VDBP is immobilized on a substrate, for a time sufficient for free and albumin-bound Vitamin D in the sample to bind to the purified VDBP, thereby forming a test sample Vitamin D-VDBP complexes; optionally removing any Vitamin D not bound to the purified VDBP from the test sample; contacting the Vitamin D-VDBP complexes with a known amount of free labeled Vitamin D, for a time sufficient for the labeled Vitamin D to equilibrate with the Vitamin D-VDBP complexes in the test sample; determining the amount of labeled Vitamin D bound to the purified VDBP in the test sample, and calculating the amount of bioavailable Vitamin D in the sample from the subject based on the amount of labeled Vitamin D bound to the purified VDBP in the test sample.
 3. The method of claim 2, wherein the substrate is beads or a solid surface.
 4. The method of claim 2, wherein the free labeled Vitamin D is labeled with a radiologically detectable tag; a fluorescent tag; a luminescent tag; or a colorimetric tag.
 5. The method of claim 1, wherein the vitamin D insufficiency treatment comprises administering a compound selected from the group consisting of: calcitriol; dihydrotachysterol; doxercalciferol; paricalcitol; cholecalciferol and ergocalciferol.
 6. The method of claim 1, further comprising detecting the presence of a vitamin D binding protein variant genotype in the subject.
 7. The method of claim 6, wherein detecting the presence of a vitamin D binding protein variant genotype comprises detecting the presence of a Gc1F, Gc1S, or Gc2 protein variant.
 8. The method of claim 7, wherein detecting the presence of a vitamin D binding protein variant genotype comprises using chromatography, mass spectrometry, antibodies directed against the specific variants, or genotyping the subject's DNA.
 9. The method of claim 8, wherein genotyping the subject's DNA comprises determining the identity of the nucleotides at rs4588 and rs7041.
 10. The method of claim 9, wherein the presence of a G genotype at rs7041 and a C genotype at rs4588 indicates the presence of the Gc1S variant of the Vitamin D binding protein; the presence of a T genotype at rs7041 and a C genotype at rs4588 indicates the presence of the Gc1F variant of the Vitamin D binding protein; and the presence of a T genotype at rs7041 and an A genotype at rs4588 indicates the presence of the Gc2 variant of the Vitamin D binding protein.
 11. The method of claim 6, wherein the level of bioavailable Vitamin D in the sample is compared to a genotype-specific reference level of Vitamin D.
 12. The method of claim 2, wherein the VDBP is immobilized on a substrate via a peptide immobilization moiety.
 13. The method of claim 12, wherein the VDBP is a fusion protein comprising a VDBP and a peptide immobilization moiety.
 14. The method of claim 12, wherein the peptide immobilization moiety is selected from the group consisting of Argtag, calmodulin-binding peptide, cellulose-binding domain, DsbA, c-myc-tag, glutathione S-transferase, FLAGtag, HAT-tag, His-tag, maltose-binding protein, NusA, Stag, SBP-tag, Strep-tag, MBP, SUMO, Protein A, Protein G, and thioredoxin.
 15. (canceled)
 16. (canceled)
 17. (canceled)
 18. (canceled)
 19. (canceled)
 20. A method of determining a Vitamin D Index for a subject, the method comprising: determining total 25(OH)D concentration in the subject; determining VDBP concentration in the subject; and calculating a ratio of total 25(OH)D concentrations divided by VDBP concentrations, thereby determining the Vitamin D Index for the subject.
 21. The method of claim 20, further comprising: determining a VDBP genotype in the subject; and comparing the Vitamin D Index to a reference Index for the subject's genotype; wherein the presence of a Vitamin D Index below the reference Index indicates that the subject has a vitamin D insufficiency.
 22. The method of claim 21, wherein determining a VDBP genotype in the subject comprises determining the identity of both alleles of one or both of the SNPs at rs4588 and rs7041 in a sample from the subject.
 23. The method of claim 22, wherein the presence of a G genotype at rs7041 and a C genotype at rs4588 indicates the presence of the Gc1S variant of the Vitamin D binding protein; the presence of a T genotype at rs7041 and a C genotype at rs4588 indicates the presence of the Gc1F variant of the Vitamin D binding protein; and the presence of a T genotype at rs7041 and an A genotype at rs4588 indicates the presence of the Gc2 variant of the Vitamin D binding protein.
 24. The method of claim 20, further comprising comparing the Vitamin D Index to a reference Index, wherein the presence of a Vitamin D Index below the reference Index indicates that the subject has a vitamin D insufficiency.
 25. The method of claim 21, comprising administering a treatment for vitamin D insufficiency to a subject who has a Vitamin D Index below the reference level.
 26. The method of claim 16, wherein the vitamin D insufficiency treatment comprises administering a compound selected from the group consisting of: calcitriol; dihydrotachysterol; doxercalciferol; paricalcitol; cholecalciferol and ergocalciferol. 